For me well I hate to lose, so I can play it on immortal and win most of the time, but if I need a bit more challenge I can hop up to deity so I can swear at it a bit (especially when losing a wonder by 1 turn). I also like the map variety and exploring as others have mentioned and being the first to find some remote island or overlooked goodie hut. However it's not a true addiction I sometimes leave off for a few weeks to play something else, but it is like an addiction in that I start craving a game of it after the hiatus and them I'm back on a Civ 5 jag that can last months.
Looking forward to beyond earth Civ 5 + my favorite scifi, I'll probably never be heard from again (I just hope they fix some of the obvious dumb AI moves that no AI or human should make unless they are addicted to something nasty).