How This Affects You
Men adopt a "get them before they get you" mentality. They definitely believe in the old saying from sports that the best defense is a good offense. Result: If they suspect that for some reason they're not your Prince Charming, they will actively seek to alienate you so they can say it was their choice and not your rejection that created distance in the relationship. Bottom line: It stands to (his) reason that if he doesn't put himself—his feelings, his ego, his desires—on the line, then he can't get hurt.
What You Can Do About It
The best strategy you can undertake to deal with a partner who is motivated by fear and self-protection is to talk about this fear openly. Be warned: Men will jump through fire to avoid this conversation, because their egos won't stand for them to admit that they fear something. What you can do is deal directly with their fear of rejection.
To prepare for this conversation, you need to ask yourself some important questions:
Are you in fact rejecting him?
Are you sending him messages that say "You are inferior and undesirable"?
Are you making him so low on your priority list that he has no choice but to conclude that he is undesirable and has been rejected?
Please understand, I'm not recommending that you just admire your partner unequivocally or encourage behavior that isn't worthy of admiration. I'm saying that you must consider whether or not you are playing into his worst fears. His worst fear is that he's not good enough for you, and that you are going to dump him.
Reassure him that while there may be issues that need to be discussed, you're not putting the relationship on the line over those problems. If you let him know that you have no intention of withholding your love, affection, attention or sexual interaction in a punitive way, you can reduce his fear of rejection.