Why won't my guy friend hug me anymore?

For starters, I'm an extremely physical person and even when talking to people I've met minutes ago it feel the need to occasionally tap them on the arm or something like that (hell, I even give my teachers a hug every once in awhile). This one friend of mine knows it. He used to hug me sometimes on his own accord, and generally was okay with me hugging him by surprise (he even patted my forearms when I threw my arms around him from behind a couple times). Now it's different, I can't even pat him on the shoulder without him getting all tense and defensive. I mean, I would understand it better if he had been like that all along, but it's not like that.

I really need help, because I love talking to him, but I feel uneasy when I know I will invade his little personal bubble just by acting natural.
Why won't my guy friend hug me anymore?
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