I think too that you can move and are alive in a mirror whereas you're static in a photo. A photo is only a moment in time and can remind you of good or bad memories, like Doh says, depending on who took the picture and how you were feeling about having it taken.

I don't care for myself in pictures but I am usually surprised by myself in mirrors. I've gained so much weight over the years that I see a completely different person than I'm expecting. Twenty years can make a huge difference (sorry for the pun :-)