Why do so many guys think me being nice means I'm flirting?
One time I was in Walmart and I passed by an older gentleman (I'm in my 20s, he was in his late 40s) and I smiled at him when I walked past him because I felt like being nice and I was in a good mood. He took this as flirting and said "Don't do that! You're going to make me think you're interested in me, what would your boyfriend think?!" and he laughed, Or something like that (don't remember what he said exactly) but it really shocked me and caught me off guard.

Same thing happened at a grocery store, I smiled at a stranger (because I thought we were supposed to be nice to each other in this society) and he started asking for my number.

These situations happen kind of often, so I've stopped smiling at people or saying hi to strangers (not that I said hi to everyone or something) just as a casual friendly exchange.
They all think I'm flirting with them! Even guys where I'm just having a nice conversation, they think I'm flirting.
I'm not doing anything to make them think I'm flirting, I'm just being nice.

This really makes me uncomfortable and I'm starting to imagine how I'm going to have to act at work in future jobs. I guess I can't be "too nice"? Ugh.