Why won't my boyfriend take his shirt off?

I have been with my boyfriend for almost 5 years now and he has never taken his shirt or clothes off during sex or in front of me in general, aside from the two times we have had sex in the shower. He is thin and has a six pack so I don't know why he would be insecure but maybe he is. What is your opinion? How do you think I could make him more comfortable or willing to take his shirt off in front of me?
Why won't my boyfriend take his shirt off?
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