How do I find a problem for my book?

There are the obvious problems (he's a vampire, she's a mermaid, etc.) and then there are the more subtle differences and problems that I find make the difference between a good book and a great book. A great book encompes and reflects all the subtleties of human nature and the way we interact. If you think about it, why do some of your relationships have problems? Not because he's a vampire, or he's a criminal, surely. In real life, there are reasons for problems you can put in your book that are less extreme, more believable, and more relatable. Personally, I would rather identify with your character because he or she has a similar problem to me, not because your character's significant other might jump out of the closet one night and suck his or her blood.