Why does Montag compare feeling unhappy and mad with feeling fat? Why might this unhappiness cause him to "start reading books"?

In my opinion, Montag uses this comparison because feeling fat generally leaves one unhappy with themselves, and people can respond to that unhappiness with complacency or change. Montag has been complacent in his life, settling for what his life is supposed to be according to the society he lives in. For him, the ability to open a book and gain knowledge would be similar to someone attempting to lose weight watching the number on the scale move downward. Montag doesn't want to settle.... he wants to be happy.