I put in lots of hours on the material I love precisely for that reason. However, I put in lots of hours on the material I love and the material I don't enjoy so much because I want high grades too! In part it's because I expect myself to perform at my very best in whatever I do, and in part it's because my family/advisor/supervisor/girlfriend/peers all have very high expectations and if I didn't do well I'd feel like I let them down. Yea, it's a stupid reason, I know.
I cope with not having a life outside of the course because I couldn't be much more antisocial, I've never really had much of a social life (even in secondary school). I guess this is an advantage really, because I don't often find myself working all night wishing I could be out somewhere else. But I do appreciate this isn't the case for most people.
One thing I've seen over the last year or so is that some people actually perform better when they have something else. I know of someone who worked every hour of every day last year and burned out (kept slugging on anyway) and ended up with a 2.2 (not sure what this is in GPA, sorry!). This year they are working a couple of hours a day plus 8 hour days at weekends, going out drinking 3 or 4 times a week, participating in several societies, and they are now getting 1sts!
It seems to me that you have to find your balance. You have to discover how much work you have to do and how much you can get away with not going in order to get the grades you want. I think your first year is very important in this regard, because you should be "tweaking" your study habits in order to find this balance.
Scott