Some folks have no lives. It's as plain as that.
I have friends who literally have no hobbies, who basically work, go out and drink beer, hang out with their wives/girlfriends, and watch tv/movies. That's it. They're usually the ones who think I'm nuts for brewing my own beer.
Generally speaking, the more inquisitive people who have their own hobbies are pretty accepting of it- they're more likely to be impressed, and think brewing is a lot harder than it really is- sometimes they may think I'm crazy for that reason.
As to why I started; a couple of friends of mine bought a starter kit in college, and I helped out with making a few batches. Of course, the boob who ran the shop didn't, I think, really know what he was doing, and did ******* things like sold "Vienna Lager" kits to guys like us who would be brewing in our dorm rooms and apartments, and we were pretty disappointed when it was estery and funky as hell.
About 5 years passed, and I ended up inheriting the old starter kit. Then, another guy I knew got married and had a kid- he gave me his brewing stuff. So there I was with a whole lot of brewing equpment that I really didn't know how to use, so I decided to figure out how to make something drinkable!