I like sub games because of the sneakyness.
You can't wade in, a-guns a-blazin. You have to plan and wait. Picking the best time to strike. I find sub games much more mentally stimulating (which does not take much with my mind

)
I also like the weaknesses of subs once the moment of surprise is gone.
I also like the solo type play although that really should not be incorporated in SH3 due to the wolf-packs.
I don't have a problem playing the Germans. It is a simulation of history. I find the prospect of killing British no more or less abhorrent than killing Japanese. I would not like to kill either in real life.
I guess my "attitude" mimics many of the real life Kregsmarine in that there was nothing personal, just a duty to do.
And, of course, I never forget it is just a frickin game