Cas, too, has asked for help, scaring himself with his own meltdown, with his attack on someone he didn't want to hurt. Missing time was never a good sign.
Right now Dean wanted help in return and Cas would try, of course. He'd do anything for Dean if he could do it. Lately his capacity had been reduced but he was trying. Cas knew Dean didn't understand Sam in the same way that Cas did. He knew it was in all their interests Dean get through to his brother too.
"Sam is scared, above all. He's scared and confused and you need to keep that in mind if you're gonna talk to him. Throw too much change at him at once and he'll clam up."