I think he was insecure. It was said that the MC was actually the one who worked the hardest and made it possible for their business to start succeeding. And as time went by, his partner (Jiang Ge) started taking on more and more responsibilities and keeping him home because he was fragile and he didn't want him to be so tired/wanted to protect and take care of him, then I think at some point he just started to think less of him? Less of an equal and more of a house husband sort of thing? And as we know, the MC thought of how he stopped working as a sacrifice that he made for Jiang Ge so this might've been reflected in his everyday well being, even subconsciously. Mean while some selfish young pretty man might've started giving jiang ge attention and made him feel like a million dollar hotshot and so he cheated once, enjoyed himself and didn't get caught, so he did again and again. And so he became the person depicted in the manhwa. And sure there might've been guilt felt towards the MC but he would justify himself by telling himself that the partners aren't serious and just boy toys and the MC is the only one he truly loves and how he still takes care of him etc etc.
Probably power. When they were poor, Jiang Wenx treated He Zhishu respectfully and was loyal to him. But then, when He Zhishu gave to him the company of his old boss (which He Zhishu was supposed to be the CEO, not Jiang), Jiang Wenxu started to changed. In the novel, we can read the secretary's POV saying how being rich and acquisitively stronger messed up with his head. It's like the quote I've heard: "Give power to someone, and you can discovers their true face". So I think not only his cheating habits, but abusive attitude are consequences from his ego for gaining power. Sorry for the bad english
I always ask myself why did Jiang Ge cheat?
Clearly he loved Zhi Shu. and yes, things go down after spending so many years with your partner but what they had was special.
Was it insecurity? There were mentions of two men being bitter with each other so I thought that. Maybe it’s also boredom.
What do you think it was? Let’s discuss