Why some people choose to suffer in the relationship?

I met this guy online. He talk about his relatioship. He is no longer happy with her. He doesn't love her anymore because she cheated him not just twice. His girlfriend doesn't treat him fairly. She controls him including his social media. This lady owns him like a dog who has no freedom. Yet, he still choose to stay with her. He said he can't leave her because he don't want to break her heart and ruin her reputation.

I feel sorry for him..
Why some people choose to suffer in the relationship?
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