Why So Selfish?

I don't understand how, or why some people are so ridiculously selfish. You didn't cherish what you had and that's no one else's fault but your own. It was what you wanted at one point and it's done and over with so stop being so selfish, it's too late for regrets. Just be happy with what you have and move on with your damn life. Stop trying to ruin the happiness of others just because you're too busy dwelling on the past. The could haves, would haves and should haves no longer exists.