Family tradition in my case...
My grandpa was a submariner on a IX/D2 (U-859), torpedoed a few miles before reaching Penang by a british sub.
My dad never got to know him and over the years became a little obsessed with him (still keeping his EK II and uniform knobs) and subs in general and I guess that was inherited upon me.
Besides that, most of the mentioned above: Das Boot movie, and some of the american sub movies, early sub games like Sub Battle Simulator, Silent Service and so on...
and reading books of course.
Stay silent...
joerd