He has a fatal illness which is his reason for joining the Nameless. He would much rather die on the battlefield than in a bed waiting for illness to claim him.

After disobeying orders for him not to sortie, Kurt learns of his deathwish and extracts a promise from Serge that, for as long as Kurt commands the squad and they have need of him, Serge will live, fight, and not die until he is given the permission to do so. Tearfully, Serge accepts the conditions.

(Source: Valkyria Wikia)