If I'm on my way home and I see a dude on a bike I feel compelled to make him work to stay ahead, or pass him. Its fun to outpace someone on my nice light bike all dressed in biking clothes. Its more fun to be wearing socks and sandels in work clothes with a backpack riding this....
Especially so when said dude is riding his titanium MTB all dressed in bike gear and climbing a hill.

Road bikes are even more fun...