Guy's Behavior

Guys, Why is he behaving like this?

I have a crush on a guy. I suspected he had a crush on me too. Sharing music, being flirty, he would smile every time he saw me. Likes teasing me and joking. We laughed together a lot. He would look at me a lot and even friends commented on how they noticed how he was with me.
suddenly one day he went out with a girl. ( I want to note that in that date he chose to eat the things he know I love and he has always teased me he would never eat) The date took me by surprise as I never saw it coming. He has since seen her again but as a group, not on their own.
my question is that if he has found a girl that he likes I will accept it however not only he denies having a girlfriend but his behaviour towards me has changed. He no longer smiles at much, he does not sit next to me as he used to. He seems to have lost his spark. For someone who is out with a girl and on his phone all the time , does not look happy. He says he is alright, fine but I can tell he isn’t.
Why when he should be happy going out with a girl he seems unhappy and why has his behaviour towards me changed?

Updates
+1 y
We have been friends for a year, flirting with each other 3 months later. We have held hands and gone to movies together but have never called it dating as such, just taking the flirting up a level. but never said “I like you as more than a friend”. Then out of the blue he goes on a date. I feel jealous but try not to let him see it. On the night he went out I sent him a couple of very flirty emoji’s…. I apologised the following day. We hugged. And suddenly his behaviour towards me has changed.
Guys, Why is he behaving like this?
Post Opinion