He stopped smiling when he saw me?

My older cousin has a best friend that I’ve known since I was five years old and we were pretty good friends, we used to hang out a lot after school and play with each other. Now, I see him sometimes when I pass by my cousin’s house to say hi and he asks me questions about the sports I’m doing etc. but it’s been a while since I’ve seen and spoken to him. We attend the same high school now and I am currently a Freshmen while he is a Junior, and now just recently I saw him walking down the hallways with this girl in which I have no clue who she is. He was smiling and talking to her when we suddenly made eye contact, his smile dropped as soon as he saw me as he stared at me. To ease the tension in the air, I smiled and said excuse me me and walked past him, but I’m wondering, why was his reaction like that? Did he not want to see me or something?
He stopped smiling when he saw me?
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