Our line for what? I've got one line for biting sarcasm, another for threatening glowers, one for preemptive gentle remonstrance, others for non-deadly force and one thick red and black one for thermonuclear annihilation.
Sarcasm is for when I want to make someone feel small and insignificant but don't want the situation to physically escalate.
The threatening gaze is for times when "I'm ready, I think you're a problem and I can take you right now," is the desired message.
And so on.
There are things other than life and death that are worth fighting for. If someone really wants a piece of me but wants to beat me up disengagement might not be an option. In that case I'm going to get into it anyway and figure it might as well be on my terms. So I'll take the cheap shot and try to stop him very quickly, preferably without him knowing exactly what happened except that he hurts. I will not use techniques that have a reasonable chance of crippling or killing.
Then there are the seminar *******s. You're working with them or demonstrating. They try to see how your steel is tempered. Or they're being jerks and bullying other participants. People like that might not have any shame. If that's the case, and if saying "Get out of here and don't come back" isn't quite the right thing (those of you who've been there know what I'm talking about) sometimes you have to explain to him in language he can understand that yours might not be bigger than his, but it's big enough to make him really sore really quickly. Not *crunch*splat* but *bang*ouchouchouchouchouch*
For the really big stuff it has to be serious - "immediate and otherwise unavoidable danger of death or serious bodily injury resulting from the unlawful use of force against you or an innocent third person". If it's a stranger whom I really and truly believe is in danger of getting murdered, raped or similar I might respond. If it's my wife, my immediate blood family, or a few close friends I will use the threat of deadly force followed up immediately by deadly force if I don't see immediate surrender or the bad guy's back disappearing over the horizon.
One of our pets? Legally there's no justification unless there's another crime like a robbery or burglary involved. Emotionally it would be hard not to come to the defense of one of our faithful dogs.