This is an interesting point. I was looking for information about what kind of sailors became U-Boat men and came across this interesting article written by Timothy Mulligan a couple of weeks ago. It's entitled "German U-boat Crews in WWII: Sociology of an Elite". You can find it in
The Journal of Military History Vol. 56 no. 2 pp. 261-281.
Anyway, he discusses reasons why the U-Boat arm of the Kriegsmarine continued to fight with such enthusiasim even after suffering such severe defeats.
If you don't have access to the journal, I can email you the .pdf. Just PM me.
And please excuse my spelling errors--I'm a horrible speller and and it embarasses me
.
Cheers,
J