Why is being nice to strangers considered weird?

A kid in my class today was asking if anyone had change for his single bill and everyone else ignored him but I remembered I had dollar bills in my bag so I started rummaging for change for this kid and my friend looked at me weird and was like, “You’re too nice.”
And I was like, “What?”
And she goes, “He’s a jerk and you’re doing him a favor.”

I mean I didn’t know him…he’s never been mean to me and he told me thank-you for the change.

Should I have ignored him like everyone else?
Is that what’s socially acceptable?

I just didn’t understand why I was made to feel uncomfortable by my friend and the rest of the class (they stared at me) for helping this guy.