Why can't I just tell them to go fuck themselves?

This is the question I ask myself on occasion.
Despite individuals frequently deserving this treatment most of the time it isn't done. The interest weighing against it is to strongly ingrained in my psyche. Considerations like burning bridges, proprieties, balance of power, shock. When objectively considering a situation I usually say I don't think I'd deal with that very well. I'd probably tell them to go fuck themselves, which is why I consider myself mildly volatile. The truth however is I underestimate my control. Most likely I will wish that I would have told them to go fuck themselves when I had the chance. I guess I'm a little bit of a real life internet tough guy. Mostly becuase I weigh my self control way to low, and my ability to react quickly far to high, in addition when examining a situation objectively I don't take into account the affect of that action on the relationship I have with people, most of the time I don't want to damage a relationship with anyone beyond repair, only in the most extreme situations and I can count them on one hand will it get to that. It usually takes such an overt and absurd aggressive stance on their part that no matter what my reaction is the relationship cannot be salvaged at all. This is probably a good thing however, why can't I just tell them to go fuck themselves? Especially if they fucking deserve it?