Eternally popular (just like its many immortal supernatural beings), brotherly bond between Damon and Stefan Salvatore formed the foundation of the show — mainly because of their rivalry and how different they both were.
However, upon closer inspection, the two polar opposite vampires were not so different after all. They had several traits that overlapped, and their actions and decisions in life also went in similar directions. The Salvatores were brothers, after all.
They Were Vampires Who Turned Into Cured Humans
Despite their initial animosity, Damon and Stefan went through many of life's milestones together. With Katherine's blood in their system, they were turned into vampires, and they transitioned into being immortal at around the same time.
For a century and a half, the brothers lived as powerful vampires, but they were not destined to stay that way. Both of them got the cure, albeit under different circumstances, and became cured humans. Damon got to live out his human life but Stefan sacrificed his.
They Both Fell Into Katherine's Trap
As young human men, Damon and Stefan were enchanted by Katherine's beauty and her charming ways. With a combination of compulsion and flirting, she had both the Salvatore boys in her control, and they had no option but to fall for it.
She turned them against each other by making them compete for her affection, and Katherine's romantic gestures led them to their eventual death at the hands of their own father, while she escaped from the other tomb vampires.
They Both Had To Do Terrible Things For Bad Guys
Throughout the show, the brothers and Elena encountered villains who were much bigger and badder than them, and sometimes the gang couldn't stop the villain. On many occasions, either Damon or Stefan had to give up their autonomy and become puppets for them to protect each other.
Stefan had to turn off his humanity and become a Ripper for Klaus to leave Damon alone, and Damon did the same for Sybil when he got caught by her in the vault in the Armory. They served their enemies for a while and committed some vile acts in their servitude, but got out eventually.
They Were Ready To Sacrifice Themselves For Each Other
When audiences first got introduced to Damon, he swore to make Stefan's life hell and the two didn't seem to be on good with each other at all. This changed slowly but surely over the seasons as they started saving each other from tight corners.
In the end, Damon and Stefan were ready to sacrifice their own lives for the other brother. Stefan was always a noble character, but when it came down to it, Damon stood ready to die with Katherine. He got outsmarted by his brother who finally did burn with Katerina so that Damon could have his dream life with Elena.
They Both Had Trusted Best Friends
In the years that Damon and Stefan were unable to see eye to eye, they made connections with other people who understood them better. Stefan found Lexi, who made him into a more humane and sane creature. She was his closest friend, and he spent many good years with her.
Damon made a connection with Enzo, even if it was under bad circumstances. They stuck by each other for a year before Damon left Enzo in the fire. He also found a kindred soul in Alaric Saltzman and Liz Forbes.
They Loved Elena
Predictably, both the brothers fell for the sweet and generous Elena, with whom they both ended up having relationships. While both of them had seen her in Mystic Falls when they returned, she first had an intense relationship with Stefan when they met at school.
Slowly, her affections turned towards his bad boy older brother Damon, who loved her back. Both of them ended up together and lived long, fruitful lives as humans after taking the cure.
They Flipped The Humanity Switch At Times
A debatable perk of being a vampire was the ability to switch off humanity, which both Stefan and Damon did multiple times. Stefan had flipped his switch right after turning, unknowingly, as he coped with being a vampire, his brother hating him, and his father's death. He then flipped it in the 20s, and then at Klaus' behest to save Elena and Damon.
Damon was mean and didn't care much for humans anyway, but he had also switched off his humanity, like Stefan. The most notable times were when he had to abandon Enzo for Augustine, and when he had to collect souls for Sybil.
They Killed Each Other's Friends
With vampire territory comes murder, and even being brothers didn't stop Damon and Stefan from killing each other's friends. Damon used Lexi as bait in Mystic Falls and staked her to prove to Liz that he was fit to be on the founder's Council.
Many seasons later, Stefan killed Enzo, not once but twice, and the second time was permanent. This was also a time when Stefan's humanity was off, and Enzo's death broke many hearts, not just Damon's.
They Died And Came Back To Life Several Times
Deaths were hardly permanent in The Vampire Diaries, and both Damon and Stefan had died and come back to life several times. Both had gone to the Other Side in season 5 when Stefan's heart was ripped out, and Damon crashed his car into the Mystic Grill. Stefan made it back but Damon did not.
He eventually did come back, and then both the brothers were killed a few times but resurrected by Cade's immortality spell. They also lost their souls to the Phoenix Stone. They had been to hell and back (quite literally) because the Salvatore brothers only left the planet on their own .
They Both Committed Horrible Crimes
The powers of mind compulsion and super speed made it very easy for the Salvatore brothers to commit crimes and erase minds, which is what they did throughout the show. As a Ripper, Stefan brutalized and killed hundreds of innocent people, so much so that he was known as a serial killer at one time.
Damon controlled women and abused them. He also murdered people, with and without his humanity, and compelled bystanders to forget. They had done a lot of wrong in their lives, and Stefan actually got to experience the consequences of when he got cured and people started ing.