The overwhelming majority of humans do, in fact, want to be human, much to the annoyance of the transhumanist minority.

Putting that aside, though, I see what I think is a different problem, though perhaps I'm overgeneralizing from my own motivations. Human endeavors tend to feel worthwhile because they are a challenge. Assuming that we do develop the ability to self-modify, recursively improving our physical and mental abilities, I worry that things will seem better and better -- until suddenly they seem worse. When anyone can be as strong or as fast as they want, there will be no such thing as athletics or martial arts. When anyone can be as smart as they want, there will be no such thing as puzzles or games. Etc. When all the hard questions have been answered, what will be left, except wireheading?