How come if someone smiles at or compliments someone of the same gender it's considered being nice, whereas if they do the same to the opposite gender it's viewed as flirtatious?

why do we consider nice, friendly behavior with someone of the same gender to be flirtatious if done with someone of the opposite gender? if we compliment someone of the opposite gender it may be seen as a “come on,” that you’re hitting on them. yet to do that to someone of your gender is not taken that way.

if i tell a girl i like her outfit or her hair looks good, it’s chummy, but if i tell a guy that his hair looks good or if i smile at him, it’s flirty. why?