SO is upset that I've changed

8 months ago, I had a beautiful baby girl and as you may know, once you become a mother your whole life changes. Your outlook on life changes. Your interests change. You change. Before having her, my SO and I used to party and drink all the time, and now, he's upset because those things don't interest me anymore. I don't drink anymore (I'm breastfeeding) and partying isn't fun to me anymore. He doesn't understand where I'm coming from. I suggested we do something else, but he says "he wants his best friend back." What can I do to show him that I'm still his "best friend"? I'm 27 and he's 25 btw