I wouldnt say humans are cruel, but most of us are selfish or self-centered. For example, you only want to run into your ex with a younger/hotter girl because of your ego. You want her to see that you can do better than her, and that your breakup didnt affect you. I am self-obsessed too, and I dont think its a bad thing, but sometimes we should all step back and try to look at things from someone else's perspective