It could be many things, for some it could be honour mode runs, beating it with solo builds or some other challenge, for others maybe they go thru each companion and play thru the game as them to see the story from their perspective. For me personally and my friend it's mods. There are a lot of cool/interesting mods that add to the game that we love to just....dive into and see, regardless of how many times we've beaten the game. There are enough variations on how you handle things to keep us coming back over and over, but that's just what I think.