umm I think it's because of some anger,or he fell in love with another girl even though he had his own gf.Sometimes even due to lack of love,or desire for sexual variety.Even to take revenge too .Even due to situational factors and opportunity.
You mean <em>"Why do [some] </em><em>people</em><em> cheat in relationships?"</em>
Well an analysis revealed eight key reasons: anger, self-esteem, lack of love, low commitment, need for variety, neglect, sexual desire, and situation or circumstance.
<em>Or the world is and humans are just terrible LoL</em>
