Why is he unhappy or even a little angry when he talks about his ex-girlfriend

Breaking up must be an ex-girlfriend. What makes him unhappy? The biggest possibility is that the woman broke up with the man. At this time, men recalled what was wrong with their happy and sweet life in the past. Why did she break up? The woman said, "Let's break up." After a series of conversations, the boy knew that she had a new boyfriend outside, and the girl had been lying to him. What an impact this has had on the boy. He is very sad, so every time he thinks of his ex-girlfriend these days, he will be unhappy and even want to kill her.