About two thirds of the way through the novel, Beatty forces Montag to burn his own house down with a flamethrower. Rather than be sad or angry about having to do that, Montag feels the way that he felt at the beginning of the story. He is happy to be burning everything to the ground.
"And as before, it was good to burn, he felt himself gush out in the fire, snatch, rend, rip in half with flame, and put away the senseless problem."
Montag is happy about burning his house down, because it is a cleansing act for him. He is purifying his life through fire. His wife's obsession with the TV and headphones and gossip drivel were sources of pain for Montag. By burning all of it to the ground, he emerges refreshed and renewed in mind and spirit. That's also how the story relates to the legend of the phoenix. Out from the ashes arises a new and more pure entity. In this case, it's Montag.