Dean is the best brother, mother, father…you could ever ask for. Think about this for a second. He found Sam lying in the hallway unconscious, running a nearly fatally high fever and he just FIXES it. Dean carried all 6’ 4” of him back to their room, gathered enough ice (presumably from a machine in the hallway) to fill half the tub, and put Sam in it. Pushed him under gently, forcing his body to expel the heat. He waited, no doubt terrified, for Sam to come out of it. When Sam wakes up, gasping and in shock, Dean is there to steady him. Sam is so confused and cold and scared that he gets angry, but Dean just explains why he did it. The look on his face and the tone of his voice gives away his fear, but he just wraps Sammy up in a thick towel and soothes him. Dean, one of these days someone needs to sit you down and take care of you. You deserve it.
Sam doing these trials?
That is Sam taking care of Dean.
Sam refused to let Dean be the one to go into the trials on a suicide mission. Sam wanted to prove that Dean isn’t the grunt of the duo, that Dean isn’t the one who always has to sacrifice.
So these trials are Sam’s way of taking care of Dean.
And it worked. Because it made Dean realize that they’re in this shit together. It made Dean trust Sam. And that’s maturity. Dean grew because of Sam’s sacrifice.