First Night-Night Alone

I put my son down in his crib to go to sleep by himself for the first time tonight. He cried when I put him down but I scratched his back for a little while and he stopped and sat down. I said "Night night" and I left. He was lying down as the door closed. He hasn't cried yet. It's been about 20 minutes. I've been crying since I shut the door. I don't even know why.