Why do some people not add me back on snapchat?

I’m new to my school, and I would say I have a sizable portion of my grade added on Snapchat (around 50 out of around 150 students). Whenever I look at people’s stories and they mention/tag someone from our grade (because it’s their birthday or some other reason), and most of the time it’s someone I know, I add them, because why not. I’ve recently noticed though that a lot of these people never add me back. If it’s someone I don’t know or don’t really talk to, I understand, but a lot of the time it’s people who I literally have classes with, people I know know me too. I’ve never really thought about it that much until now, but it’s starting to annoy me. Can someone tell me what might be the case?

Why do some people not add me back on snapchat?
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