There's nothing like nagging and moaning to drag you down, my first wife was North of Ireland Champion and to be honest in the end I'd just like to have stuck a sledghammer in her face, so I do sympathise, however I think for the most part, grand dragons aside, it's just that after a while people do get stuck in a rut.
My second wife and I have been together nearly ten years and although we are both very busy people and sometimes don't see a huge amount of each other we always make an effort to do special things together, go places, see people, try new and different things.
We've both got a very high sex drive which is good as we are evenly matched, but again I think the key is to never let it go stale and to just try to keep the romance and spark alive, and have fun.
To stave off problems we set aside a nagging hour each week where we can fully vent our frustrations without the other one getting upset, and nagging is banned at all other times. We then also tell each other how much we appreciate the nice things the other has done all week, so it always ends on a positive note.