Because they did'nt know when they were fated to die. It might be that day, or the next. Or ten years later. So they fought as hard as they could and protected themselves whenever possible. They just did'nt bother worrying if they were going to win, lose or die. Because after a certain point it was out of their control anyway.
Determinism is not the same as defeatism. And the Norsemen's philosophy is no different from a Christian saying, "God works in mysterious ways", or a Muslim stating, "If God wills it".
I disagree, there is no difference. If nothing can change your fate then nothing you do will matter. Whether you know that fate or not would also make no difference.
I find it far more motivating (and likely) that our actions define our outcomes.
Anyway, to contribute to the thread:
"It is easier to find men who will volunteer to die, than to find those who are willing to endure pain with patience." - Julius Caesar