Damon was always possessive of Stefan but was too egoistic to accept it
Damon doesn't like Stefan loving anyone more than or even as much as him. Hence all the gf sharing(Katherine), gf snatching(Elena), killing Zach n his pregnant wife(their last living family. Stef refused to go on a road trip with him since he had built a life for himself there so he destroyed that new life forcing him to flee), killing Lexi(she stopped him from joining Stef in the war n if not for her, he'd have personally come to get his humanity back so he left her on the terrace and killed her fr when they met again).
He's very possessive of his baby bro hence that dialogue to Klaus "He already has a brother...not to be territorial or anything" n got soo angry thinking Stef betrayed him to save Klaus's life. He can't even stand him giving a 2nd chance to their own mom. He was actually disappointed to know she's alive, rescued her only to get Stefan's humanity back, tried killing Julian knowing very well they r linked n his mom'd die too, said to her face "It's no skin off my back if u r still linked when I drive a stake through his(Julian's) heart. I'm only here coz Stefan's a mamma's boy n I don't want the silent treatment for the next century". Dude was kinda relieved when she finally died...
The only reason he approved of Steroline was coz Damon knew he won't ever love her more than him, that he'll always be his 1st priority. He hated Valerie's guts coz she won't take any of his bs n would try to convince Stefan to prioritise himself.
Damon always loved Stefan (albeit in a toxic way) n wanted to be Stefan's no.1 priority but he could never truly admit it. So he tried to focus more on competing with him for girls, to be chosen over him for once coz even his parents preferred Stefan but truth is even for Damon, it was always gonna be Stefan and he was so damn jealous of Stefan being so loved by everyone, even by himself that he spent his whole vampire life making him think he isn't really that loved after all, convincing himself n his lil bro that he doesn't need him...
But the whole castle of lies came crumbling down when Stefan actually died for real in the finale. After losing Stefan, his eyes lost spark too and that spark never came back, not even on seeing "the love of his life" Elena back from coma. He never once smiled genuinely after his death (while every other character did), he looked so lost even after getting to live his dream life with his dream girl coz that was never his actual dream in the 1st place, it was something he tried hard to convince himself n everyone else but now it was too late. He couldn't even kill himself coz that will make Stefan's sacrifice go in vain so he lived his human life the way Stefan wanted him to but in reality, he wasn't living – just surviving, which is clear from his expressions.
In the afterlife, he left Elena's hand the 1st chance he got and ran straight to his little brother, the one he always wanted deep inside. He looked relieved n at peace for the 1st time in his "long, happy human life" when he embraced his brother again coz now he could finally be himself without all that pretence of hating him n loving some girl more than him...they could finally have an eternity of misery peace together without anyone's interference which could have happened earlier if not for Damon's ego and stupidity. That's the real tragedy of The Salvatore Brothers.
