It's because both of them were awful people.
The main character had no self esteem so he constantly fished his girlfriend for compliments. The low self esteem also lead to upon seeing her with another guy, he concluded she was cheating. He may have also been stalking her, to be able to "coincidently" just catch her with a guy like that. He has a nasty personality too: with how he just burst in and accused her and his present actions of harboring a grudge for years and creating an intricate revenge plan.
Also pointed out is the girl had a big head since she was pretty and treated the guy horribly. She riddiculed his hobbies never told him she loved him. Although, the reason she "lied" about cheating is because she was upset that he accused her, which made her fall out of love with him on the spot.
Honestly they were both terrible, so ironically, they're perfect for eachother.