Why do guys stay in relationships with women they don’t love?

I was video chatting with my best friend earlier in the late morning/early afternoon, and he told me that he’s not sure if he loves his girlfriend but he hopes he does, and he told me he has sex with other women while in his relationship, and him and his girlfriend are having ups and downs lately. I told him that it’s normal to have ups and downs in any relationship but there are exceptions and rules, and the fact that he’s 100% sure about her says something in itself. So why does he stay with her? They’ve been together for nearly a year
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Last night I couldn’t sleep, and he messaged me when he got off work and when I replied, he asked me why I wasn’t asleep and he told me to drink warm milk which I did, and he said if I don’t pass out to go on a walk or run at the park. And right when it was all done I fell asleep
Why do guys stay in relationships with women they don’t love?
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