When you go to replay a game, why do you do it? Do you go for 100% completion even if you've already done it before? Do you play on a greater difficulty? Have you ever tried to replay an entire series back to back? How soon after finishing up a game the first time have you returned to replay it?
Sometimes if I really like a game, I'll replay it just to experience it again (Zelda games, for example). Other times, it's to clean up achievements, alternate paths, or other things that can't be unlocked in one playthrough. Or with Pokemon games, I'll get the urge to use certain Pokemon on my team and I'll replay the games.
When I replay something, it's to do a 100% completion run, and I only play on a higher difficulty if it unlocks something extra or if I receive an achievement (Arkham Asylum, Ghostbusters: The Video Game). Normally I don't replay games if I've done a 100% completion before but there have been exceptions (Mario Kart 64, Pokemon Snap, Pikmin, Star Wars: Bounty Hunter). I don't think I've played an entire series of games back to back. It's obviously harder to accomplish a feat like that depending on what the series is (some series' contain 3 games, some 10 or more). I also wait awhile before replaying something, usually more than 6 months.
I hardly ever have the attention span to complete games in the first place... so I usually play half way through, and then restart because I've forgotten a lot of the game, and then don't get as far as I did the first time. lol But assuming that I did finish the game, I usually do replay them expecting to beat the main story, and find stuff that I never did the first time.
I have absolutely no time whatsoever to play games during the week, and not enough time to play games properly at the weekend.. :( Mostly I'm just playing my Pokemon games recently, though, and I rarely have much of a problem with replaying through them- for the sake of a different starter Pokemon, playing as a girl (I don't even know why), or just out of boredom. However if it's a Pokemon game with an actually decent post-game, I rarely restart, as I'm usually too focused on achieving something.
