Why do (some) guys withdraw after being emotionally vulnerable leading to hot and cold behaviour?

I've gotten used to the ebb and flow of his moods (and he withdraws less and apologises) but I'm still left feeling sad after big moments of intimacy and feeling closer after him showing vulnerability? How do I reassure him it's OK? Or initiate contact to show its fine, I won't bring it up so he doesn't have to feel awkward.

Why do (some) guys withdraw after being emotionally vulnerable leading to hot and cold behaviour?
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