Why won't this relationship work out?

I won't give too many details, but in my book two "ghost hunter" type characters fall in love with each other. The girl falls for the boy, but although he also likes her, he tries to avoid her and rejects her. I need some creative and imaginative and secretive reasons for why a "ghost hunter" would deny himself a relationship with another "ghost hunter" even though he likes her... I have some ideas of my own, but I just want ideas from people who don't know the full story yet. Thanks!