The monster was created accidentally by Dan. Dan stole Elise's Genome Reorganizer Machine with the intent on using it to destroy all vegetables. Dan activates it in a broccoli field, creating the Broccoli monster.
The monster tells Dan that the Broccoli gods command him to eat Dan in order to become one. Dan escapes by telling the monster he has sauce in the car that will make him taste better. He then flees to Chris and Elise's house where they discuss options on destroying it. The monster finds him and Dan flees again.
Dan leads the monster past a store full of starving vegans due to the vegetable shortage Dan created. The vegans chase after the monster, finally catching up to it. The monster pleads for Dan's help as the vegans eat it. Dan just walks away with Chris and Elise.
Since Dan is his father (in a way) he must eat him in order to become one with broccoli. His relationship died with Dan as he tried to run away from him.