How many times have you smiled at yourself in the mirror while you were washing your hands only to discover that you had something stuck in your tooth? To make things worse, you'd just spent the whole afternoon smiling at everyone you met, and chatting people up because you were in such a good mood. And, no one told you. Not one person cared enough to clue you in to that little green "something" stuck in your front tooth.
How did that make you feel? Like crap, no doubt. Embarrassed? Hurt? Perturbed?
Is it easier to just say nothing and go on my way hoping someone else will tell them? Yes.
I would never want anyone to feel that I didn't care enough about them to clue them in.
Heck, I tell complete strangers if they have toilet paper stuck on their shoes. Or that their skirt is tucked into their nylons and their behind is hanging out for the world to see, and I once even told someone they had a little blood on their pants....(if you know what that is code for). I consider it a random act of kindness when I say something to a stranger. I consider it a sign of respect when I tell someone I know.