why does montag cross his lawn to eavesdrop on clarisses house?

Montag wants to know the enlightenment Clarisse experiences in her house. Clarisse's house is full of cozy light, enlightening conversation, and genuine affection. For Montag, entering his house was:

"like coming into the cold marbled room of a mausoleum after the moon has set. Complete darkness, not a hint of the silver world outside, the windows tightly shut, the chamber a tomb world where no sound from the great city could penetrate."