First of course because of the many many pretty things you can find, second because of the people you meet. It's interesting how different people are: Some time ago, I was at a fleamarket, looking at a pile of beautiful old fabrics and table cloths. While I was having a closer look at everything, the woman yelled at me: "Don't unfold!! I can tell you anything you want to know!". Now - what's the sense of selling stuff if you're not even allowed to touch it and have a close look at it? I didn't say anything and just moved on to the next table.
The purchase of the little etagère above was a much nicer experience though: Me: What do you want for this? Her: Uhm, I don't know... (feeling a little uncomfortable) Maybe three euros? Me (feeling lucky): Alright, I'll take it. Her (a bit disbelieving): Really?