Friendly vs flirting?

I know there's a very thin line between being friendly or flirtatious.. So when do you know when you cross the line? In general I'm very friendly to the people who look more approachable.. I like to smile, say hello, make eye contact for a few seconds, with family and close friends we hug, kiss on the chick, etc.. For some reason with females and some males is a natural thing to do, with others may seem like I'm flirting with them.. Is this right, if a female is too nice would think she's flirting with u?
Friendly vs flirting?
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