How to understand guys?

I decided to put things behind me and reconnect with old friends one them dealing with my ex. When we were together he was obsessed with me, even afterwards. I put a stop to his obsessive behavior afterwards, a few months back. 8 months later I compliment his Instagram post and he ignores the shit outta me. He respond and likes everyone else’s comment and totally acts like I don’t exist. I honestly wanted to know what I did wrong?
How to understand guys?
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