I get the sense that in our culture, serious voluntary suffering is unacceptable. However, there is at least one exception. When you are suicidal, you are required to keep living. It is socially unacceptable that you die, even though dying would end your suffering. You are expected to endure however much suffering you have to in order to save something of great value, your own biological life.
Maybe that's the only thing that really matters to us -- human life. But specifically, the life of humans we know personally. We might value all human lives, in principle, but where we say in an effective way "human life matters" (by urging people to keep living no matter what) traces out where, effectively, we encourage human life. We tend to only know people personally whom we like (we might sometimes also hate them, simultaneously, but we must like them to get close). So that's our effective moral circle. We are champions of love and care for people close to us whom we like. And we encourage people who are close to us, whom we like, to do their best to live forever, to put up with whatever awful feelings they have to to make that happen. That is a real commitment we call for.
But if we are altruists, we should care about distant people, or even all sentient beings. So then, if we are really altruists, we should be willing to suffer as much as we would have to avoid our own suicide, if that suffering is necessary to remain true to our altruism.
And if we are theists, we should care about God, and all that he cares about. Then, if we are really theists, we should be willing to suffer as much as we would have to avoid our own suicide, if that suffering is necessary to remain true to our theism.
Altruism and theism don't necessarily call for us to burn ourselves out, or to sacrifice our biological lives (although when truly called-for, they do). Altruism and theism are ways of life, ways of seeing things, sensitivities, the ability to take into account the existence of all people, or all sentient beings, or God. Usually they recommend that we be most productive in the long term, which usually means that we should practice moderation in pouring ourselves out, and that should be our default assumption.
We each have a biological life, which we are expected to fight bitterly to preserve. It doesn't matter how hard you work, or how little you work, with the time you have, you are expected to preserve your biological life. That basic life is what you fight for. Altruists and theists have additional lives. No matter how much or little you work as an altruist or theist (depending on the resources you have available), altruism is a life, and theism is a life, and these can end if we are not careful. These lives are really others' lives, of God and other sentient beings, and for their sake it is acceptable to suffer, just as it is acceptable to struggle to not end our own biological lives.
(God loses a life each time one of us is lost. So that he does not have to lose lives, each of ours, we should hold onto theism.)