I think it's just that we're creatures of habit.
I love Saturday nights in with a few beers and a take-away of some sort.
I roast a nice big lump of meat every Sunday if I can, and if I don't, I feel like it wasn't a proper Sunday!
I didn't really have roasts much when I was growing up so it's not carried over from childhood, it's just that I love them!
Oh and I live on my own but I still do them (so I don't have to share!!)
But I do really enjoy the rare occasions I get to cook for someone, especially a roast.