Why do deaf people talk strange?

I apologize if this sounds rude, my roommate is completely deaf and he talks strange. I feel bad asking him, because I feel as if it's insulting him. What is the reason that deaf people talk weird?
Update: kaydid: I actually did learn some sign language for communicating, but he is able to read lips so it's not communication that's the issue. I was just confused by why it seems that he almost speaks with a lisp.
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