I don't think it's fair to say Dean gave 0 shit's about Castiel's suicidal confession. He's the one who sat down with Cas when he realized he was upset, asked what was wrong, and listened openly to what he had to say. Their conversation was just interrupted before he could respond. That episode touched on suicidal ideation, but it was more specifically about escapism and the way Cas was avoiding the responsibly of dealing with his mistakes by hanging out with the Winchesters. By the time the case had wrapped up and they could talk again Cas had already overcome that particular struggle by watching Sam confront the psychic about their own experiences with escapism. And as soon he worked that shit out Naomi was in his head pulling his strings, and he was no longer was acting like Cas. You can see Dean still making gestures to cheer him up (offering to let him ride shotgun), but by then the moment to resume their conversation had passed and Castiel was pulling away from him because of mind control.
Even if they had dwelt on that conversation longer, I don't think it would have been in character for Dean to sit him down and extoll to him all the things he has to live for and how much everyone would miss him if he were gone. Dean has been the one standing on a ledge more than anyone, so he knows that hearing those things don't do anything to change the way you feel. Instead, he to chose to stop pushing Cas about heaven and allowed him the space to figure things out, which is what he really needed. And I also think you can make a strong case that his "I need you" line in Goodbye Stranger was a belated response to what Cas said in Hunteri Heroici. And before any of that he gave the same sentiment in S7, with his "I'd rather have you, cursed or not" line.
My god, those seasons were so gay. Truly peak Destiel.