Conventional wisdom suggests, of course! The worst answer you can get for asking is, “No”, and as such you’d be no worse off than having not asked. That is simply not true, because one is obviously not hitting on a stranger when professing one’s love, and as such risks jeopardising their existing relationship with this person.
So, well, “No”, and possibly they then immaturely go off and tell everyone that you hit on them (then add in whatever to make you sound creepy) and to avoid you. In any event, there’s a more evolved reason than rejection, shaming and embarrassment to keep one’s feelings to oneself.
It’s about not burdening another person to carry the weight of your feelings for them just so you can get it off your chest when there are clear indicators that this person would not, for whatever reason, be interested, able or willing to reciprocate. If you know it would not change a thing except the satisfaction of having vented, but with the possibility of losing the proverbial bird in hand.