In Runs In The Family (Reprise), Pony says "If you hate us so much then you should just leave" to which Darry replies "Oh you've got no idea what I gave up for you"
Which is a somewhat odd response. Think about it. If someone told you that you're clearly unhappy and shouldn't stick around, your response wouldn't be Ugh I've put up with so much.
But consider, someone tells you "I know you don't want to be here", and you say "I've fought hard to be here. I obviously want to be here because if I didn't, I would have given up long ago." Darry goes on to draw attention to his hands and points out how hard he pushes himself, saying "I do it for you, ponyboy, and for your knucklehead brother. So we can live in this house. So there can be food on the table."
His entire point is that he loves them so much. That he's angry and scared out of his love for his brothers.
Then Pony responds "you're not dad, Darrel, and you're not mom either, so stop trying to be" which changes the whole direction and clarifies Pony's perspective. He's already said that he thinks darry hates him (and soda) (and resents them for keeping him there) but now he's reminding darry of the responsibility he's taken on, throws it in his face that he's not doing a good job, and is clearly mourning the loss of his oldest brother who has been replaced (in his mind) by a domineering faux parent.