Why does Two-Bit grin when Ponyboy picks up the glass?

In the scene, Ponyboy is picking up pieces of glass from a broken bottle. The sequence can be found in the last chapter of the book. Ponyboy is hanging out with Steve and Two-Bit when a group of Socs comes by and starts talking trash at Ponyboy. Ponyboy does not hesitate. He picks up a bottle, breaks it, and starts at the Socs. He threatens them right back, and the Socs drive off in their car. Two-Bit is shocked to see Ponyboy act this way, and Two-Bit even comments that he does not like seeing Pony act in such a way.

Ponyboy, listen, don't get tough. You're not like the rest of us and don't try to be.

Like Johnny, Two-Bit wants to see Ponyboy "stay gold." Right after he says this, Ponyboy starts picking up the pieces of the glass bottle. Two-Bit smiles because he realizes that Ponyboy has not changed. He is still the good kid that they all know and love. The smile is an amused smile mixed in with a little bit of relief.

I didn't know what he was talking about, so I just went on picking up the glass from the bottle end and put it in a trash can. I didn't want anyone to get a flat tire.