why do this kind of person exist?

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My officemate has already a girlfriend for 5 years.As of this moment they are far from each other because the girl is working abroad.Despite of their mutual love,the boy still fools around and having other girl on his side.Everytime we have a chance to talk to each other,he is always telling me that he misses and loves his girl so much.I always ask him that why is he doing that kind of thing if he really loves her girl?I always tell her that maybe he should let go of her for he is not worthy for the girl's love for him,but he does'nt want to.I feel bad for her girlfriend and at the same time for the girl who've seriously love him.I also had a chance to talk to his other girl and she told me that she was happy to be with him and if the time came that his girlfriend arrived,she is willing to let go.I already told my officemate about this,that he should'nt treat a girl like that but still he does'nt listen.I really don't understand why do this kind of person exist.