I think for me, it's the incredible tension and emotion in me that this game generates. I find myself getting excited when I spot the first inklings of a ship/convoy/task force heading my way. Then there's the tension of tracking the prey without being seen... getting into just the right spot to take your shot.
Then there's more tension after you fire... as you wait several minutes hoping and praying that your calculations were correct ! Sometimes, when I hit a target, I feel like jumping out of my chair and shouting "YAHOOOOOO !!!" while tossing things in the air as the explosion kicks that plume of water into the air.
Then there is the opposite tension.. the tension of being the hunted. No other game scares the pants off me more than this game. You're diving for cover.. then you hear them approaching... their engines churning through the water.. getting closer.. closer... then.. there's that haunting "ping ! ping !" followed by the sound of their engines getting closer....closer.... then "Splash"... "Please god ! Don't let those depth charges be near me.. please... please...."
Then.. if you escape... there's that huge feeling of relief when you surface. You get to breathe easy... until you spot that British plane diving down on you...
This game is an emotional roller coaster for me and I LOVE the ride.