Mesirath Nefesh is widely considered a virtue. It is treated by Musar-givers throughout the world as though it is the most important virtue after fear of Heaven. But what is it based on? There are many Mitzvoth requiring us to give. But it usually is a minor financial sacrifice, often based upon one's means: give an animal sacrifice; give Ma'aser; give half a Shekel. Even when one is told to put forth physical effort, it is along the lines of helping in a way that isn't self-detrimental: help your enemy's donkey with its burden; help your fellow who is being attacked (though we are not required to endanger our own lives for this); help your brothers in arms (although you'd potentially be putting yourself in harm's way, it's also in your own self-interest to be a part of the military unit protecting the people - including your own family).
But is there a Mitzvah - or at least a source in Pesukim - that putting one's own interests aside to accomplish something greater, even if it requires self-sacrifice, great effort, or some heavy loss, is a virtue, let alone the great virtue that it is so often said to be?