Why would a successful 26-year old guy go back home?
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To deliver the most epic Rickrolling ever.
A mouse popped out of his mom's.......um, let's go with "air conditioner," and she's traumatized.
To convert her to Scientology.
To help renovate the house.
To give them all anthrax.
He realized that he was getting older, and he never properly flashed his parents.
He simply had to show his parents his used condom collection.
He thought, "You know what, I feel like visiting home for a month."
And of course:
Gordon Freeman with a gravity gun.