While this was an extreme example of heroism, such self-sacrificing behavior is common enough in our species to raise a challenge to the TOE. Kin selection doesn't explain such behavior, because non-kin are often the beneficiaries. Reciprocal altruism might explain it, but that seems to be a stretch when a young man who has not had an opportunity to reproduce sacrifices his life for others. Perhaps this behavior is evidence for group selection, but most evolutionary biologists seem to believe that group selection plays little, if any role, in evolution.
There is no way to know very much definitively. But we do know that intelligence and altruism seem to coincide. For instance, dolphins have been known to save the lives of imperiled humans. Why? We don't know. But then, we don't even know why we do the same things.
Why dive on a grenade? It's not something someone cognizantly thinks about. It's a reaction.
Why did this dog save another dog? Who knows... All we know is that they do, and on that basis we cannot rule out some sort of evolutionary reason.
"Reason obeys itself; and ignorance submits to whatever is dictated to it" -- Thomas Paine
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