I think he just didn't want to play what as expected from him, the role of the war hero.
I like it when he tells Lt. Werner in the very beginning to not make photos now but at the end of their mission because everybody will have beards ... and when the sub comes back to La Rochelle we see the young sailors with bears, looking grim, pissed off about what has been done to them and not too happy about the military pomp. Like the KaLeun they have quickly become old men.
My favourite character is the second officer
. When they dive deeply for the first time during a test alert and the sub begins to crack he talks with the greenhorn Werner about how deep they can go and how the water will crush them if they go too deeply ... and he totally enjoys to fu*k with him and frighten him even more.