this isn't an evolutionary explanation, but for me a girl who shows when she is sad feels genuine. very many girls will hide their emotions and fake being happy all the time. I personally don't like being around these kinds of girls.
it's also a sign that a girl isn't a batshit crazy bitch (again, like very many other girls) because the loco ones will take it out on other people when they aren't in a good mood instead of like the girls in this thread that instead turn to depression or self-loathing
also like the other anon said... it also does kind of pull on that moe urge to cheer her up/protect her