Guys, Why does my boyfriend only kiss me when we're alone?

So I noticed lately that my boyfriend doesn't kiss me in public, only when he's over at my house and we're about to make out and more. It makes me think that he only kisses me for sexual stuff which I hope isn't the case so why does he only kiss me when it's just us and when we're getting in the mood?
Guys, Why does my boyfriend only kiss me when we're alone?
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