What's the big deal with hickeys?

Both me and my boyfriend are 17 and have been going out for 11 months. Last night I came come home from his house and my mom saw there was a hickey on my neck and FREAKED OUT. Like she said I looked like a major slut, but I don't understand why. IT WAS ON MY NECK, that's it... So why?
What's the big deal with hickeys?
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