My 17 year old son recently got his first girlfriend and I'm sad about it. I don't understand why

My son is shy and quiet and I assumed he wouldn't have a girlfriend until he became more confident, but one of the only girls he's friends with asked him out a few months ago. She come over sometimes and they cuddle and watch movies on his laptop. Seeing him put his head on her shoulder and hold her makes me feel sad and I don't understand why. I've also gotten sad when I walked into the living room and caught them kissing and once when he was cuddling with her and was resting his head on her breasts. I also accidentally picked up his phone one day thinking it was mine and his wallpaper (a picture of his girlfriend kissing his cheek and him grinning) made me emotional. I don't understand what's wrong with me. Why do I feel sad about this? via /r/Parenting http://ift.tt/2y0ZrhT