This song speaks to me in a very personal level. Being the person who've experienced failed relationships quite often, I manage to stay 'cool' with my exes because I'm not that person who held grudges on people because I don't want to go sleep thinking about what can I do to make my exes suffer or plotting a revenge against them. I prefer to move on with my life by being friends with them.
Because I can't stand the feeling of knowing someone is mad at me or they hated me for whatever reasons. So I choose to stay friends and at least I know we are not in a bad term. This doesn't apply to romantic relationship only. It also applies to my the friendships I have with my friends. I think it is better to put the past behind us and I don't care how much they've hurt me, if they want to be friends, I'm okay with that because after all that we've been through, I know we're cool!
P/S: I know some people prefer to stay away from their exes but what's with the bad blood? Who knows you might need help from each other in the future.