Why are “nice guy” boyfriends so mean to their girlfriends?

I don't know if the title makes sense but my boyfriend is a very nice and friendly guy. He makes sure everyone feels seen and appreciated… except for me. He told me he wants to put kindness into the world. But then he treats me like garbage. He yells at me, insults, gets angry and sarcastic with me. When I cried to him, he told me he doesn’t want to deal with me. But then talks about how important mental health is with other people.
I always try to be kind, clear, and respectful but he’ll ignore me for days if I cry or do something small he doesn’t like. For example, I was put on a sedative and he got angry and hung up the phone on me because I slurred my words (even though he knew I was on meds). Little, harmless things turn into problems.
I don’t understand why everyone else gets treated nicely but not me. I’ve seen this with my parents too. My dad is friendly with others but mean to my mom. How can nice people treat their partners so badly?
We haven’t been together long, only 3 months, but I’m planning on leaving the next time he yells or makes me feel bad about myself. I’m sick of being treated this way.

Why are “nice guy” boyfriends so mean to their girlfriends?
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