For Selwyn, Naegali wasn't just an old alpine guide. He was a father figure, someone who represented a symbolic reincarnation of Selwyn's schoolteacher. This was important to Selwyn, because his teacher's placing his hand on the young Selwyn's head is the sole memory he has of his childhood home. That being the case, Naegali also stands for Selwyn's homeland and more specifically his attachment to and memory of it. So when Selwyn loses Naegali, he's experiencing once more the loss of his homeland. Not surprisingly this is very hard for him to take.
In fact, Selwyn's response to news of Naegali's death suggests the presence of trauma lingering in his psyche. It plunges him into a depression so deep that it almost causes him to be discharged from the military. Yet Selwyn still has the presence of mind to be able to describe his deteriorating psychological condition. Appropriately enough, he likens his crippling depression to being buried under snow and ice, which is of course exactly what happened to Naegali on the Aare glacier.
Now that Naegali is dead, Selwyn's past has been irretrievably lost; it was buried along with the old alpine guide. When Naegali disappeared into the crevice so too did Hersch Seweryn. By the same symbolic logic, when the body of Naegali resurfaces so too does the memory of Selwyn's repressed identity as Hersch Seweryn.