Well I suspect people just fall into the "More power and bigger will make people like me!" trap - I keep getting people telling me I'll sell my moped and buy a motorcycle because I want the power. (Yes, I feel a terrible urge to buy an overpriced racing bike and plaster myself against a brick wall at 100mph just to look cool.... gee won't I look cool in the full body cast after the skin grafts? :)
Some people buy mopeds for fun and then get teased by someone or other... or just don't think they look cool enough... and they end up sitting in the garage. In my case I don't care what people think of me, I ride the thing because its inexpensive and its fun and its the perfect city riding vehicle. Who needs 100mph when the average traffic in the city goes at about a quarter of that?
Besides - I have fun responding to some of the comments on the street. Had one guy shout out "Great bike loser" to which I replied "I'm riding, you're walking, who's the loser?" (Of course it helps when making comments to be riding past faster than they could run after you)
I've also had some positive feedback.. just WHEELING my project moped (a jawa '87) down to the workshop where I'm going to be taking it apart, I had two people stop me and ask if I wanted to sell the thing. With the way gas prices are going, I think we're going to see more moped riders popping up and that they will become a lot more popular. (probably a bad thing... no more rebels without a clutch! ;)