what thought is johnny trying to communicate to ponyboy

Johnny encourages Ponyboy not to waste his youth looking for things he does't have. Johnny gets rather Zen about life using Frost's poem as a metaphor. He tells Pony that life is fleeting and nothing is permanent hence "Nothing Gold can Stay". Johnny encourages Pony to see the goodness in people like Darry instead of wasting their relationship.

Johnny also asks Pony not to become hard like Dally. He wants Pony to stay true to himself.