Control freaks, the lot of them.

Personally, I'd rather do it myself so I know it's done right. What if they pinch a tube and blow it up, or put the wheel on crooked, or tighten the skewer too much, or not enough? Or set the bike down on the wrong side, or drop the wheel and scratch the skewer?

Really, have you seen the way some people wrench on their bikes?

No, thanks, I'll take care of it myself.

Must be I'm one, too.