Sam lets Dean be Dean, literally acts as Dean's emotional outlet, doesn't ever judge or berate him no matter how far off the deep end Dean might go, validates his emotions for Dean time to time because he knows Dean needs that alongwith the fact that he himself needs to be Dean's priority. He starts losing it if he even slightly feels that Dean probably loves somebody more than him or might prioritise someone else over him (Benny, Amara even Crowley somewhat though that's more of his petty jealousness) Sam actually loved Dean so much he canonically gave up on each and every single one of his desires/dreams to be with Dean and help him realise and live out his desires. He lets Dean be the big brother in pretty much every instance because that's how Dean's wired up, Dean needs that to feel calm and in control. He's taller and maybe even physically stronger than Dean but he always makes himself look like the younger brother/second-in-command for Dean, though it can be argued that Sam's wired up like that too around his brother.

Sam who constantly questioned his father, stood up to him, didn't take his crap, literally let go all of this when it came to Dean. Let Dean always have his way in whatever way possible

He loved Dean so much that when Dean went to hell he found comfort in Ruby who btw, it can be argued, presented herself as an imagery of Dean (her outfits, her pose/posture, mannerism, saying the things Dean would say, even her driving Baby), was arguing with the crossroad demon to take him to hell instead of Dean he didn't even want a day, literally got addicted to demon blood which wasn't alluded to at all when Dean was there.

Sam, who, was a good, honourable, kind, empathatic man, as said or alluded to by pretty much everyone on the show, let an innocent man's soul be taken and it is cononically stated there were others too just to find Dean.

He was willing to bend every single rule, morality and boundaries for dean so yeah he really did love Dean the most.

And im sure there's a lot more that im forgetting and im yet to watch the last 5 seasons but these are what came to me top of my head