Why did my guy friend like my sister's instagram selfie?

There's this guy friend I hooked up with a couple months ago, but I stopped meeting up with him for personal reasons. We coincidentally have a class together this semester, and became friends again. Recently, my sister told me that he liked one of her selfies, but he didn't try to talk to her or continue to like her posts. I am confused as to why he did that. Is he trying to make me jealous? Is he still interested in me? And what was he looking for on there? And lastly, should I ask him about it?
Why did my guy friend like my sister's instagram selfie?
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