We'll just get the most important part of it the way right up front: the blood drinking is a plot device.  There's no real explanation for it, no one else has it, it's mostly there to bring the two characters together and then be a kinky relationship thing. 
I knew that going into the novel and I think that made it easier for me to accept it but ymmv. 

As far as the actual story is concerned, I actually really enjoyed this one.  Could the two of them use some actual therapy to deal with their emotional issues?  God yes.  But I will say that I didn't overly feel like this story relied on a love cures all mental problems outlook.  The ML doesn't stop needing some sort of release on the pressure he's dealing with. They do develop a somewhat acknowledged unhealthy codependency on each other. But they're also still young and I can see them growing and healing over time, together. 

All in all, a pretty decent novel with some sexy blood drinking scenes if that's your thing.