Thanos of Titan was one of the sons of A'Lars, his father who is an Eternal, and his mother Sui-San, the last remaining Eternal from Uranos' followers. Together, A'Lars or more commonly known as Mentor, and Sui-San rebuilt an Eternal civilization on Titan known as the Eternals of Titans. They began to repopulate Titan, with Mentor as ruler of Titan, and bearing children of which only two are named - Thanos and Eros.
Eternals physically look like the average human being but when Thanos was born, his skin was purple and tough like leather because he had a mutation called the Deviant Gene, that came with the Deviant Syndrome. Thanos was the only individual known to suffer from the Deviant Syndrome, and aside from his physical differences, he was stronger and smarter than other Eternals. Thanos' psychopathic side showed early on. When he was a kid, he enjoyed drawing pictures of dead animals and as he become a teenager, he took a liking to the idea of death. In his teenage years, Thanos was smarter than any of his peers and he began to conduct experiments on animals to learn more about them while trying to understand his genetic mutation. He befriends a mysterious girl who encouraged him to push himself, even to go as far as killing fellow Titanians. With her as his confidant, Thanos starts to kill and experiment on other Titanians, recording the first murders in the new Titan's history. After 17 kills, he finally kills his mother Sui-San and in hopes of finding answers about his mutation. He falls in love with the mysterious girl but she rejects his advances.
After the murder of his mother and the rejection of his love, Thanos left Titan to join a pirate ship and travel the universe, vowing to never kill again. Over time, he lost his sense of humility and forgot how to feel emotions. He travelled from planet to planet, consummating with natives and leaving them and the children they bore him because he felt no attachment of any sort. In a conflict with the captain of the ship, he kills the captain, reminding him of the sweet taste of death, and he decided to embrace his need to kill. After the kill, he takes over the crew and returns to Titan.
Upon arriving, he is visited by his old friend, the mysterious girl. She gives him a chance to prove his love to her by demanding that he does something big. He sets sail and travels to all the previous planets he had been to, to kill all of his children. When he went back to meet her, she told him that in order for her to truly love him, he must become a supreme conqueror and destroyer of worlds. Realizing that none of his crew members is able to see this mysterious girl, she finally revealed herself - she was the physical form of Death itself. At that point, Thanos was already too much in love with her and would do anything to please her. Hence he began his chaotic journey by destroying Titan.
The destruction of Titan was the beginning of Thanos' obsession with conquering worlds, which later changed to a skewed ideology of saving the universe by destroying half of it, and all of it done for his lover, Mistress Death. Thus, his hunt for cosmic power began.