For me: I find always found it fascinating that in 4 short years the hunter became the hunted and lost by such a high amount it was practically suicide to leave port. But yet they went.
Also the German navy was the least nazified fighting force of the German army, and many skippers fought with honour. But in the end this sums it up:
Winston Churchill (1943):
"The only thing that ever really frightened me during the war was the U-boat peril".
And we see the first real submarines come to live: Type XXI and XXIII.
Plus it's a game and I love a challenge
P.s. Read "Iron Coffins" by Herbert A. Werner