I do this, simply because i resent being told how to feel, or to like a movie. It's why I dislike romcom soundtracks (now its the funny part, laugh, now its the sweet part, say aww. No Country For Old Men was excellent as there was no music to prompt me). I try to catch movies before I see much of the trailer, or before all the 'wow it's great' hype. Having said that, I enjoy reviews, and read them before and after I see a movie.

look it's a media thing.
The media love to build up a smallish film ( underdog story).
Then once this has happened. the media love knocking them back down again.

This is also true when the medai deal with anything at all.
Let it be a story, people etc. anything.