When do you stop fighting?

Life is a battlefield, plain and simple.
You argue with yourself over what is right and wrong.
Mother-daughter fights are bound to happen, especially in your adolescent years.
Sibling rivalries are inevitable, and bitching out your best friend from time to time is a clause that is written in permanent ink in the "friendship contract" that is created through silent mutual agreements.

Then there are more serious fights, like wars, politics, cancer, and other illnesses.

But when do you know when enough is enough?
You push yourself to the limit believing that you can win, but what you have just been fighting a losing battle to begin with?

Sometimes the battles are not even worth fighting.
Which ones to you choose to sit out of, and which ones to thrust yourself into?

I suppose it is just a matter of the heart.