My baby is growing up

Tonight Collin is sleeping over at his friends' house. Tommy and Jimmy live right behind us and Collin just loves playing with them. They spent the night at our house once before a few months ago but this is the first time Collin has spent the night at their house. He was just so excited about going over tonight. I picked him up at 3:30 and as soon as I walked in the door he was asking to go to his friends house. We came home and packed his overnight bag and spent some time hanging out (and counting the minutes until it was time to go). When I walked him over he went right in to hang out with his friends. I was just about completely forgotten the minute he was through the door. Before I left, I did manage to get a hug and a kiss from him, but there was no trepidation about me leaving whatsoever. It was "Bye Mom. See ya tomorrow. I know, I know, I'll be good. I promise." In other words, I'm a big boy and I'm playing with my friends now. Please leave me alone. He wasn't acting like a little 3 year old. He was acting like a well adjusted much older child and I was a bit conflicted about it. One the one hand, I was very happy and proud that he had such good friends that he was completely comfortable leaving us for the night. On the other hand, it was another reminder of how big my baby boy has gotten. Another reminder of how fleeting childhood is. And another reminder that I have to be willing to let go a little bit at a time so that when he's really ready to be on his own, many years from now, I'll be ready too. He doesn't think spending the night with his friends is a big deal, but it is for me. Its another baby step on his road to independence. He takes these steps without even thinking about them. I, however, notice and agonize over every step.