Why do people take social media breaks?

I'm not trying to be funny, I genuinely want to understand why sometimes social media affects someone's mental health.

I've had social media for years and I don't think I've ever been affected by it. I scroll a couple of minutes a day, I see people post memes or pictures of their lives but it's never caused me anxiety or negative feelings. I guess I don't take it too seriously?

A Facebook friend just put down her phone number in case anyone wants to reach her; said she used to be a lot happier before social media so she's going to delete all her socials.

I'm totally happy with people taking steps toward working on their mental health, but I guess I'm just a bit confused as to how other people's posts can affect a spectators mind. Are they in a dark side of the internet that I don't know about?

I can't imagine anyone else's life having power over me so I guess I'm just confused.