How Do I Know A Good Thing

I don't understand why my rommate wants to be with me or why he is there for me and basically taking care of me.  is he a saint or is there something seriously wrong with him.  There are times when I adore him he couldn't be more perfect and he can do no wrong.  The rest of the time I think he is abusive capable of major manipulation.  it's the black and white thinking.  i understand that now, that's why i can't trust myself even with relationships.  I could love a monster and run from an angel and i wouldn't know what I was doing.