I'm picky. Not just with books but with everything. I'm the type of person who will walk around in a shop, see something I like, pass by it looking for something else that's better, and then end up returning to that initial pick. For this reason I like shopping alone. I can take my time with my indecision.
I'm the same way with books. It's rare for me to buy a book the first time I see it (unless its by a favorite author, or the cover and blurb are smoking hot). I take a lot of time to choose the books that I read. Back in the day when all I read were paperbacks, I would choose the top ones that I wanted to read. I'd:
1. stare at the cover to see if it was calling to me
2. analyze the blurb
3. read the short excerpt
In the end I'd put all the books on my bed, close my eyes, shuffle them around and then pick one at random. Not scientific, but it was fun.
With my transition to e-books, the process changed a little. The analysis of the books occur with the same criteria, but there is no physical shuffling of them. Because of the hunting I do to get the book in the first place I tend to choose books which I know I'll like. I don't have time to waste on those that I won't. Hence the high ratings.