Feel free to flood this with the endless amounts of things that Dean and Sam have done for each other.

I'll start:

Dean devoted his entire life to keeping Sam safe
Sam gave up his chance at normal because he felt happier with Dean
Dean sold his soul for Sam
Sam taught Dean+ https://twitter.com/Baeksmileaddict/status/1330600972180213772

to stand up for himself and against their father's rule because he's more than just a soldier
Dean decided to wait for Sam to turn (Croatoan), planning to kill Sam and then himself immediately after
Sam sacrificed his humanity so that he could embrace a darker side of himself+

in the pursuit of finding a way to get Dean out of Hell, avenge him by killing Lilith
Dean left Lisa and Ben behind because he would never be able to fully commit to her because Sam comes first
Sam was able to fight off Lucifer's control because his desire to save Dean, and+

the world was stronger than Lucifer's possession
Dean and Sam chose to kill death rather than be their own downfall because Dean couldn't bring himself to kill Sam, thus unleashing the darkness
Dean was prepared to die on the chance that his suicide attempt could bring Sam back+

That's it from me. There are so many more things, so feel free to add them anyone. Lol.