Because nothing is constant when it comes to women that's why. One woman says cat and that means "I like you and am flirting" another says cat and it means "I own a dog". Guys have to sit around and try to interpret what women really mean because there's no consistency. It's all feelings and mood and temperament and alignment of the moon with Jupiter and ...

Just like your grocery store encounter. Long time advice has been that men should meet women at the grocery store.
How to Meet Women at the Grocery Store | Dating Tips - Match.com

But that's not true every time is it? So now it falls to interpretation. We're back to the old guessing game. Is she flirting? Maybe she isn't. I'll have to try to be sure.

So to sum up. Why do guys think being nice is flirting? Because they aren't sure and sometimes being nice does mean flirting.