why was he acting like this?

So i went to a dinner party and my ex went as well. We were together for 3 years and broke up 2 years ago. On my way there i was running late and he called asking where i was and text me asking what i wanted to drink. Well when i got there i had to sit either in front of him or next to him because i told a friend of mine which is his friend as well to save me a seat. Knowing him, i thought he would be really loud and be really happy like he used to act after the breakup, instead i was the really happy one. I kept laughing and smiling and talking to everyone, while he sits in front of me looking down and sad when everyone else is laughing with me at my jokes. But when im away and walk back to the table i see him talking to other people looking happy. He talks to me once and awhile on instant messenger and he knows what ive been up to but when i saw him he didnt even ask me anything or talk to me... whats with this? btw- he has a gf and i expected him to bring her, but he didnt! thanks in advance