Why did my best guy friend change once I started seeing someone?

Bensic
This is my first post. Thus I dont know if I'm on the right topic but I hope I am.
I am a 20 years old girl and I have a best male friend who is a few years older than me. We are on the same wavelenght and we feel totally comfortable around eachother. our friendship is one of the greatest thing that we have. We are really close, people think that we are dating since we're all the time together, we hold hands etc. and our friends and families say that we look cute together and that we will end up getting married. But in fact, we've never considered being in a relationship. (i must confess I used to like him a lot when we first started hanging out, but I realized it was not possible and I just dropped the idea and stood there to watch him date super duper hot ho*s)
Now he has a girlfriend. She is amazing, and does her best to make him feel special. But I relly dont understand his behavior. When we hang out with friends. It's my hand that he holds even when she is there. He kisses my hand in public, never lets me leave soon, and always puts me first (which makes her hate me and that is totally normal).
Our friends told me that he seems to be into me, because he's never been this caring with anybody. But still, I never thought of it that way. Until last week. I started dating someone. A really brilliant guy that he knows. And all of a sudden, he completely changed. He shows up most of the time when I'm with my date and shows lots of jealousy. Two days ago, he hugged me in front of him like he hasn't seen me in years, tucked my hair behind my ears and carressed my face. I was kind of shocked because lately he started behaving less friendly. Yesterday he even kissed me. I told him he shouldn't have and he apologized for it. But now everything is weird between us, he finds really silly reasons to call or text me. It is getting awkward.
What am I supposed to think? Does that mean that he feels something for me? Or is it just being jealous because I'm seeing someone else?
Why did my best guy friend change once I started seeing someone?
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