Why do guys treat you like their girlfriend but then run for the hills when you admit to having feelings??

I (24F) have been dating this guy (27) for 3 months. We have never put words to what we’re doing. He intro’d me to everyone important in his life, he often comes over at 6am to see me before work because he misses me, he plans dates to cook dinner for me, and he invites me into his daily life. But the other day when I finally was ready to tell him that I really liked him, he immediately shut down and told me he wasn’t interested in anything serious right now. Now we’re in an awkward place where he knows I like him and he’s backing off and treating me like some casual fling. How do I make sense of this??