First of all, no, you do not need a reason to reread the Harry Potter series. However, if you'd like a reason other than "because I love it so much it hurts," then these are extra justifications for plowing through the seven books. Again. Do they get old after the 27th read? I can tell you from personal experience that no, they do not. Here's more fuel for your (Goblet of) fire.
And embrace the nostalgia of reading the books for the very first time.
Unacceptable.
Like this J.K. Rowling-approved theory about Dumbledore and Death.
Like how the beautiful relationship between Harry and Dumbledore comes full circle at the end of Half-Blood Prince.
And you just want to hang out with them again.
Other books just don't measure up sometimes.
Or because you're happy and you want to feel even happier.
And get really mad at all the details the movies left out or the glaring differences between the two.
And the lessons age with you. You'll gain even more from the books as you gain more experience with each passing year.
Because letting your imagination run wild now and then is good for you.
Mischief managed.
It's the best way to tide you over until your trip.
And you know just the place to find it.
Even though it's SO hard not to tell them what's going to happen.
And you never will be too old for Harry Potter.