As much as I hated that Pee cheated on Mook, I think I would have been able to forgive him in that situation, (although, I still want to punch him for how he handled it afterwards). After some time has passed anyway.
1. Mook and Pee were basically high school lovers, true that some people just know that someone is the one, and don't need to explore. However, what if you are just settling, and think you love that person, but, in reality you're just attached to the idea of loving them?
2. Also, some people are so sheltered and in their own little worlds, they do know how good they have it until it's gone or almost gone. I know this sounds stupid and silly, but how do you know that person is the best person for you, if you've never experienced the worst?
I have an ex-boyfriend, we're good friends now. We were together most of my high school years. He constantly cheated on me (I didn't find out til towards the end of our relationship), mostly because I refused to sleep with him (I was like underage dude). Anyway, he would cheat on me because there were girls willing to give it up, when I wasn't. After we broke up he started dating a girl he tried to stay faithful to, but she never trusted him (even if he had never cheated on her), she constantly checked his phone, called his friends and family to make sure he was really with them, called his work, checked his clothes. He thought maybe it was because she had been cheated on in the past, but it wasn't, she was cheating on him (and was scared he would cheat on her). Anyhow, our relationship had ended on bad terms and we didn't speak for years until we both became adults. I ran into him because his sister is one of my close friends. He told me if he could do it all over again, he would have never let me go because of all the girls he's dated, I was the one he treated the worst (in terms of fidelity and lies), but I was the only one who treated him the best (no checking up, paranoid about who he talks to and etc).
One reason I admired Mook was because she walked away and not play into the "Ok, I'll give him time to grow up." Not many women could do that. She walked away from the love of her life because she loved herself more (and there is nothing wrong with this).