I'll Tell You What I Want (What I Really, Really Want)!
And no, I don't think it's for you to get with my friends. That sounds rather icky, actually.
But I also don't think it's money or sex or power, which are usually the first three guesses (and understandably so). It's not even fame per se, though it's closer to that.
People want attention. They just want attention. It doesn't have to be fame, but just attention, from mom's hug to the internet flame war someone starts. From whom this attention comes and for what is a matter of great difference between people and can change in a person's life (perhaps many times) but I think it all comes down to that. That's why people become rock stars or college professors or PTA members or serial killers or adulterers - the attention. The sex or money is ancilliary. Even if they sneak around, there's attention in some way. For example, you wouldn't steal something if no one would notice it missing.
Just my thought. So I'm going to go get some attention now. Oh, wait, I just did!