why does johnny's remark about the sunrise remind ponyboy of robert frost's pome?

The sunrise is an important idea because it always sets. Like Johnny, the sunrise is beautiful for awhile but then must end. Johnny mentions the sunrise and this, of course, makes Pony think of the impermanence of beauty. The fact that something or someone is precious means that it cannot last. Frosts poem perfectly captures this meaning. Thus Pony thinks about the poem,

"Eden sank to grief,

So dawn goes down to day.

Nothing gold can stay."

He has to let go of Johnny, just as a tree must let go of its leaves or the dawn must let go to day. Yes , it probably does remind Pony of his mother. He had to let go of her because, like Johnny and the poem, the best things in life must end.