There are so many reasons!
I think first and foremost would be the challenge. I think the human body is just an absolutely fascinating system and I know that I will be seeing such a variety of amazing recoveries, unforseen losses, and all that will come between. It just gives me a chill!!
Also, depending on where I work, I will always see something new and different. To have a job where I do the exact same thing day in and day out would drive me nuts! (I do realize that the basics will be the same day-in-out--paperwork, politics, etc. but the patients and the issues they are facing will be different)
Nursing is also very flexible. My DH has a job which may mean moving around. I will almost always be able to find a job as a nurse. It may not be exactly where I would prefer to be but a job none-the-less. Also the flexibilty in hours. With two young kids, I can work relief and not put anyone out if the kids get sick or something comes up or I can work nights, the possibilities are endless-which is what I love!
Finally, the money. For a relatively short learning curve (two-year ADN for me), I will make good money. With two young DD's, that means I can help my husband with college, weddings, etc. and hopefully squeeze in a few family vacations since we have only had one since we were married and we both love to travel.
I am sure I could come up with more, but I think these are the most important to me.