Why is Montag mesmerized by the conversations he overhears as he hides in the woods, observing the hobos around the fire?

First.... Montag is mesmerized by the fire, as it is unlike any fire he has known.....

That small motion, the white and red colour, a strange fire because it meant a different thing to him.

It was not burning; it was warming!'

He saw many hands held to its warmth, hands without arms, hidden in darkness. Above the hands, motionless faces that were only moved and tossed and flickered with firelight. He hadn't known fire could look this way. He had never thought in his life that it could give as well as take.

Even its smell was different.

Second, he was mesmerized by the silence.... something that he was unfamiliar with in his world of white noise.

There was a silence gathered all about that fire and the silence was in the men's faces, and time was there, time enough to sit by this rusting track under the trees, and look at the world and turn it over with the eyes, as if it were held to the centre of the bonfire, a piece of steel these men were all shaping. It was not only the fire that was different. It was the silence. Montag moved toward this special silence that was concerned with all of the world.

Thirdly, Montag was mesmerized by the free speech.... the ability to talk about anything freely and without fear.

The voices talked of everything, there was nothing they could not talk about, he knew from the very cadence and motion and continual stir of curiosity and wonder in them.