I'm a perfectionist, maybe it's my biggest flaw, so when I get a reminder every time I zone of how much of an area I have left to complete I always have the nagging urge to get that one last heart, or find that final way point. It's a nice reminder though, Guild Wars 2 doesn't force you to do anything if you don't want to and there's no rush either. Completing a zone in 15minutes won't get you any extra rewards and the journey to 100% is where all the fun is anyway.
For example, I was on my way to get the last heart in Diessa Plateau and ran across this guy laying waste to the Town of Nageling. There was a number of Charr, Norn, and Humans all working together to fend off the giant and many of them were already dead on the ground by the time I showed up. A dynamic event like this, with it's promises of loot, experience, karma, and gold is just too hard to pass up. How could I not help out? After all, I am the awesome level 33 Mesmer who vanquished the Centaur Horde from Kessex Hills. Lending a hand is my duty.
For my troubles was rewarded with a badass sword. It had +condition damage, great DPS and since I was in dire need of replacing the crap sword I had been running around with for what seemed like an eternity, well I couldn't have been happier.
Did I mention the sword was badass? Just look at the spikes.
The crazy part is that to get 100% in Diessa Plateau the Champion Giant is totally optional, and yet completely necessary. Having a "questing structure" (I don't know if I should even call it that, you never actually accept a quest apart from your personal story) that creates habitual exploration like Guild Wars 2 has done means that by venturing out to complete a zone you feel obliged to help every little villager who's running into an issue. You're a spectacular hero saving the lands from subjugation and ruin, and by doing so getting handsomely rewarded in the process.
Oh and if that wasn't reason enough, once you finally do get that coveted 100% the Tyrian Explorer's Society mails you gold, and loot, and praise. It's all win and I'm loving every obsessive moment of it.