If I was going to answer more thoroughly, here are my answers on why to start something:
1) you can work on what you like, and in a way that you like
2) it's higher beta, meaning the top outcomes are higher than working for others.
3) it is a chance to immediately start working on a high impact idea that could change society. There are other ways to work on high impact ideas, but often times you need to put in a lot of time paying dues to get to the point you can work on them.
4) if you are successful, it can be a source of mainstream fame (think Richard Branson) -- if you care about this at all.
1) you can work on what you like, and in a way that you like 2) it's higher beta, meaning the top outcomes are higher than working for others. 3) it is a chance to immediately start working on a high impact idea that could change society. There are other ways to work on high impact ideas, but often times you need to put in a lot of time paying dues to get to the point you can work on them. 4) if you are successful, it can be a source of mainstream fame (think Richard Branson) -- if you care about this at all.
Here's a summary I wrote of my YC interview that I did with Emmett back in 2005: http://areallybadidea.com/34320844