What does a guy mean when he says "Don't get too close to me" or "I don't understand how you can like me."

Because of his problems (and despite you accepting and liking him), he probably feels like his problems are going to be too hurtful or dangerous to you. He cares enough about you to not want to see you get hurt, even by him. So it's his way of avoiding letting you in to see his problems, for any number of his own reasons.
I would say he thinks his life is crap, and probably may have thought about ending it. Being from drinkers you should know he is pushing you away and not wanting help. I know you feel for this guy and he feels for you but until he truly wants to get help you’re just waiting your time with him.
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Sounds like a good guy, but really needs to learn to love himself more. Maybe you could get him into therapy. Of course bringing up the issue of therapy can be difficult. He may also need some sort of anti-depressant.
It means he's not into you and doesn't want you to get too attached
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