The two factors that determine what generation a person is born into are birthrate and economy. As the birthrate increases, the population becomes dense, creating a situation where certain ideals and thoughts can become a norm and are easily communicated amongst that generation. The state of the economy plays a similar role by dictating the intensity of concerns and social climate.
Most generations are split in half, depending on how long a particular generation has existed. The first half of each generation (we'll call them the first set) begins with a specific collection of ideals that unify them. The second half (usually the children of the first half, we'll call them the second set) of said generation extrapolates these ideals and makes them their own. One might say that they take the lessons they have learned from their parents and created a less pure form of these ideals.
Between generations (one generation is made up of first and second sets) there is a rebound effect that occurs. When the birthrate dips down, it creates a social schism. The sparse population between the two generations causes a breakdown in communication.
The schism is enhanced partly when the larger body (made up of the first and second sets), places restrictions or rules on a smaller body, creating a unified resentment amongst that smaller body. The best examples of this effect can be taken from animals in the wild. When a wild animal has been backed into a corner, whether they agree with what is being done or not has nothing to do with their reaction. Every animal craves free will and forcing something upon them enacts a fight or flight response. They also know that they must gather together in order to protect themselves against the larger body.
In the human world of social regulations, the end result is the creation of a new generation with almost polar opposite ideals (we'll call this the reaction effect). Much like Newton's third law of motion, for every action there is an equal an opposite reaction. The implications of this law vary depending on how much force the next generation perceives the first and second sets applying.

Grandparents

For most of us our grandparents were born prior to 1945. These are the parents of our parents', the ones who shaped and raised the children we now call mom and dad. Our grandparents' children, like us, stepped away from their parents' belief system to form a new generation. To understand the direction our parents headed, we must first understand what they left behind.
The actually beginnings of our grandparents generation is indeterminable due to the immigration explosion around 1900 (producing a population growth of 16.3 million) but can quite clearly be seen to terminate around 1945 (from 1910 to1920 the population grew by 14.1 million, from 1920 to 1930 the population grew by 16.6 million, from 1930 to 1940 the population grew by 9.0 million) or the end of WWII. The first set of this generation saw themselves as pioneers of a new country. Their mission was to build up the world with their minds and ideas. They had an external world view, due to large overseas communications. They thought innovatively for the greater good, turning themselves into the makers and contributors to society (example: Henry Ford).
The second set of this generation took from their parents the idea of a world view and the desire to build up their homes. They left behind the concept of innovation and the end result was the mass production of cars, skyscrapers, boats and bridges. These men and woman decided that the best way to make themselves comfortable and at home was to make their homes themselves. When their homes were threatened during WWII, society as a whole banded together for the greater good and went to war.
At the end of WWII, the soldiers returned home to their wives and settled down to create families-- unintentionally creating a huge spike in the birthrate. Thus, the reaction effect occurred.

Parents

The baby boomers are not like the generation prior. They saw how their parents' views of working for the greater good had a negative effect (hunger, disease, dirt and war) and took an opposite approach. While the reaction effect plays a heavy hand in the mindset of a generation, each generation still takes lessons from the previous generation. The first set of baby boomers took from their parents the idea of working as part of the larger world and combined it with the desire to make a name for themselves for personal gain. They knew that the only way to make money in a faceless mass of mass produced items was to create something no one else had created and capitalize on it. They sought money, fame and notoriety. The biggest example of this is the computer revolution.
The second set of the baby boomer generation (second and third children and children of the first set of baby boomers), sometimes referred to as generation x (though this is often a misnomer as people belonging to generation x are from a small set of the population born in the late sixties), took the idea of personal gain and ran with it-- meanwhile losing the global contribution perspective of their parents and grandparents. They gained the mindset of thinking and acting like everyone else.
Two things happened to the second set of this generation. The first was the huge economic downturn which hampered their goals of personal gain (thus the suicides in 1987). The second thing to happen was the Vietnam War, which put a spike in the birthrate during the early 70's. This spike cut out a 5-10 year chunk of the population during that period (generation x). Those people born in that 5-10 year period (the late 60's) tend to have views similar to the next generation rather than that of the baby boomers. This is because of the fact that they were born on the down slide of the birthrate during the baby boomer generation. The people who were born after closely resemble the rest of the baby boomer generation. Another effect of the second set of the baby boomer generation is seen in the dot com boom. The younger, less cautious, people leapt into a money-making scheme by starting up internet companies and investing in them. What happened next may become synonymous with every undertaking of the baby boomers. As word spread that there was money to be made in dot coms, more and more people joined in-- flooding the market with empty shell companies. More people were interested in getting financed than investing until suddenly everyone realized they were buying and selling thoughts and idea. Business crashed and money was lost.
But the dot coms won't be the baby boomers last attempt at making their fortunes. Due to the long gradual decline of births from 1965-1980 (as opposed to the sharp jump in 1945) and the exponential growth of technology and communications the baby boomers have a couple more chances to try. The most recent and sudden trend of soon-to-decline money-making strategies is home development (everything from building homes, selling homes, mortgages, real-estate and improvement). This long trend began with Martha Stewart and other home decorating shows and extended out to the huge up rise of mortgage companies. If television is anyway to predict the outcome of this trend, we'll see all sorts of versions of the same thing. In the next movement, we'll see the baby boomers turn to buying already established small businesses, e-commerce and prefab (Mary Kay, consulting) businesses. To predict the outcome of this you will have to factor in the second set of the generation's trend of quickly jumping on the bandwagon, the current downturn in the economy and the rise of the next generation.

Us

This new generation (generation y) is a result of the reaction effect towards the baby boomers. They've taken their parents' idea of being like everyone else and doing it for themselves first and flipped it around. They have also taken up a renewed interest in the global world (something partly handed down from their grandparents but more closely related to the effects of war). The problems we will see or are seeing is that generation Y has its goals in direct opposition to the baby boomers, who outnumber them by a couple million people. As mentioned above, the larger a previous generation is, the more connected and reactive the smaller population becomes. This is what has happened to generation Y-- forcing them to become "subversive" and "less vocal". This is best characterized in generation Y by the "aimless" (unemployed, still living at home and no careers) nature baby boomers apply to them. From generation Y's perspective, it's seen as an inability to go anywhere due to spaces, opportunities and ideas that have been closed down by the older generation.