Why he immediately leave me once he saw his friend?

We always stay in class for awhile after class end, and his friends (in the same class) will wait for him at the main area.. The moment we go out, and once he saw his friends, he will immediately go to them without even saying bye to me..
Both of his friends are girl, BUT the same girls are the one who left him to sit with me in class, just both of us in 1 row, and even if we (him and his 1 girl-friend and me) meet on the way to class, the girl will immediately leave him to let him walk with me..
It's so confusing

Why he immediately leave me once he saw his friend?
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