To set one's expectations so high that one will inevitably be disappointed by the outcome. Sometimes hyphenated as "let-down." I know you're really excited for this movie to come out, but I think you're setting yourself up for a letdown. John was so convinced that his relationship with Mary was perfect that he set himself up for a let-down in the long run.

Something that is disappointing, usually because it did not meet one's high expectations about it. Not getting tickets to the game after my aunt promised them to us is a real letdown. Sue was really excited about her new position, but now that's she's in it, it seems to be a letdown.