(Season 8 aside because, frankly, I've disinherited it and moved away to a reclusive, winter-bound, mountain cabin to forget it ever happened.)
Sheith is the most healthy, collaborative, loyal, mutually-trusting, inspiring, and loving relationship I've seen in Voltron: Legendary Defender.
From the moment they'd met they'd had each other's backs. Keith had no one; his mom had to pursue her mission's end and his father died in a fire. Whereas Shiro had everyone in his corner, yet no one at the same time; his illness forced a time limit on Shiro's life, and left everyone telling Shiro what to do instead of what he wanted to do.
Shiro was the only one to believe in Keith when no one else would; when even he had given up on himself. Keith was the only one to ask Shiro what he was going to do despite his illness; reminded him that he wasn't just his illness.
"Patience yields focus"; it's what Shiro taught Keith- to be patient even in times of chaos.
"As many times as it takes"; it's what Keith taught Shiro- that even when all seemed lost, there'd always be Keith who'd be there to guide him back.
Keith was never a lost-cause kid with discipline issues. He was a talented young man who, in time, grew up to be one of the most proficient pilots Shiro's ever seen- even surpassing Shiro himself. Yet, despite everyone's insistence that Keith was trouble and would never amount to anything good, Shiro stuck by Keith. Consistently, he'd take responsibility for Keith's shortcomings even when he didn't need to, and help Keith grow past them. He always promised.
Shiro was never a disease with limited time and unable to pursue his dreams. He was one of the most famous pilots in the Garrison from sheer skill. Even when everyone close to him told him to give up and be thankful for the time he had, Keith never chose to. He never gave his opinion and never tried to convince Shiro to take a certain path or decision.
During the Trials of Marmora, Keith only managed to unlock his blade's potential when he decided to throw it away to keep Shiro safe. Likewise, Shiro forgoes the prospect of an alliance with the Blades of Marmora in order to shield Keith from further pain. They're both willing to sacrifice majorly important things to keep the other safe.
Keith really meant it when he said "my life would have been a lot different if I hadn't met you." And Shiro acknowledges that as fact; that yes: he helped Keith grow into a great man. That together, they'd influenced each other's lives in such a way that without the other they'd be infinitely different and possibly worse off than they were then.
I respect it if you don't ship Sheith, this is just my expressing my love of it.
I'm sorry if you hate Sheith as a romantic ship or as platonic.
I love these two, great men - great characters - who love each other with all they have and will ever be.