Why are guys hot and cold?

last week I was at a party, and saw a cute guy there and he kept looking my way , and kept smiling. I smiled back, and we kept checking each other out. Even my friend noticed , and told me to talk to him but I was too shy. This week I found him on Instagram and decided to DM him and told him he was cute, and the only thing he replied to me was thank you. I don’t understand... is he shy or just not interested in me?
Why are guys hot and cold?
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