When modding Morrowind I give up fairly easily... and yet, here I am, modding New Vegas, and I'm not tired at all. When I mod Morrowind, my brain already makes up a playthrough in my head and I know everything that is going to happen. Ironically, even though New Vegas is the RPG I know the most like the back of my head, my brain is able to reset itself so that I don't think about what's going to happen during a playthrough, letting me enjoy it as if it was the first time.
Bethesda, Obsidian and the modding community are a match made in heaven.