fed up always being the "nice" person

The saying goes: "what comes around goes around" I have often wondered how true this is. I have had to deal with some pretty crabby people recently and it is really becoming difficult to keep that smile plastered on my face. I find myself daydreaming about just unloading. "You think your day is bad, well let me tell ya........." But I don't. I bite my tongue and ignore the rudeness. What do you suppose would happen if I ever did become unglued?