"But WHY is force only justified in responce to unjustified force? And why is force justified then? Why isn't ALL coercion bad, period?"
Let's get a little deeper here, shall we?
We dislike force because it is destructive. There is nothing creative about it. We all like the concept of creation of wealth - the gains from trade. Force is intrinsically destructive and NEVER results in gains from trade. There is always a deadweight loss somewhere where force has been utilized.
Now, we might say that creativity is intrisically moral, while destruction is intrinsically immoral. Equilibrium would perhaps be amoral. To which I would reply that the opportunity cost of static environments is the creativity that could have been there, and thus amorality is immorality.
Coming full circle: Force is justifiable in response to unjustified force because it causes the destruction to stop. Destruction of a destructive activity is good. It's not two wrongs make a right, though. The destructive must be destroyed by creativity. The victim may encroach upon the rights of a perpetrator only to the precise same level that he has been encroached upon, and that only until the perpetrator steps back, at which point the victim must step back also.
Let's try another tact: Encroachment is an unwritten contract giving both parties permission to encroach upon the other to an equal degree. If I step into your bubble one step, you may step into mine one step.
Why shouldn't we all be immediately upon one another then? Because the opportunity cost of being so close is too high. If I hit you and you hit me we have both suffered harm, and each of us hitting one another again has a diminishing marginal return, both in the satisfaction of hitting the other person, and in the wound suffered from the blow. The incentive to stop fighting quickly arises.