SO many reasons! most have already been named. DEFINATELY the childhood memories. I've been thirteen times since 1992. my first trip was in June and it was the summer between 2nd and 3rd grade. i've been obsessed with it ever since...and thankfully, my whole family has been obsessed as well (which means fairly frequent visits considering we live in KY)
believe it or not, i love the drive! 99% of the time we drive at night, and i'm a night owl as it is, so i love the night time travel. although, we're usually in southern GA, northen FL by dawn, and after that it seems like the trip takes FOREVER!
someone also mentioned music. just today i was decorating my living room for Halloween, and i filled my dad's 5-changer cd player with nothing but Disney World music. i swear, my heart soars everytime i hear that music!
Tomorrowland. i LOVE tomorrowland. especially at night! there's nothing in the world like tomorrowland at night!! the music, the lights, the atmoshpere...it's so beautiful!
adventureland used to be a top priority for me....but they just HAD to change it to agrabah, didn't they? don't get me wrong, i love the aladdin ride, and the themeing is awesome, but i would've loved the changes 100 times more if they were temporary.
the resorts! the resorts are one of the greatest things about WDW! i could spend a whole week just visiting resorts! my faves to stay at are Dixie Landings (now known as Riverside, but it will ALWAYS be Dixie Landings to me!), All Star Music, Boardwalk, and Wilderness Lodge. i would LOVE to stay at Animal Kingdom Lodge too...it's probably the best thing WDW has built within the past 2 or 3 years! just in my opinion at least.
the people. you meet some of the neatest people there! of course, with the good comes the bad, but you just have to overlook the rude ones, otherwise it'll ruin your day! and the castmembers are awesome! (especially one i've been searching for for over a year) though i've heard through some people on these boards (i believe...) that the quality of cm's is going down some.
the magic. everyone else in my family (grandparents, etc.) wonder why my family and i go as often as we do. personally, it's because of how it makes me feel when i'm there. i'm happy, self-confident, secure, for the most part stress-free, and being there gives me a sense of freedom. of course, there's usually a breakout in my family where we just get on each other's nerves and we start getting in arguements or whatever....btw, does that happen with every family? hopefully it's not just mine.
great question, btw! it was fun to answer!!