This is the question from the 2011 Stanford application. This is a tough question for anyone to answer without sounding sappy or trite. It is also easy to sound preachy.

This is another way of asking what you value. A value can be thought of as an emotional state. For instance, you might value the feeling of being loved or the feeling of being safe or the feeling of being important.

You will value things that make you happy.

Here is how I might answer this question in 100 words thinking about it for two minutes:

Happiness is not always successfully attained by those who pursue it using a traditional path which includes an education, a job, a home, familyand friends. I think many people fail because they believe that happiness is realized only when these things are successfully attained. I believe, however, that happiness can be found in the pursuit of these things and not merely in the attainment of these things. What matters to me, therefore, is my ability to fully engage myself the pursuit of happiness and relish the rewards that are part of the adventure itself. I want to be happy.
This could be better because it is generic, but I think the core lesson here is to make sure you connect the answer back to your values. Here is another try with more specific examples:
It matters to me that I matter. In high school it’s easy to go with the flow and not make any waves. I chose to take my own path and start my own charity to provide school supplies to six carefully selected schools in Africa, Haiti and Nicaragua. Through this effort I am able to not only learn about business and organization, but through the communication I receive from the schools and the students and know that my efforts matter. There is still about life I don’t know, I now mattering to others matters to me- and makes me happy.
Both essays are 100 words exactly.