The specifics of Ego’s origins remain unclear to Ego even to him; he eventually came to understand himself floating through the void of space alone. Eventually, he developed the ability to manipulate matter and molecules which allowed him to travel the cosmos in search of other life and dwell with other species with his humanoid avatar that was an extension of his consciousness. Disappointed and unimpressed with what he found, he decided to implant seedlings in thousands of planets, capable of terraforming into extensions of himself, and mated with numerous sentient beings in hopes of children that would carry his genetic makeup needed to carry out the Expansion.
Such A Fine Celestial
Celestials like Ego are categorized as a highly-intelligent ancient race of entities with god-like power. Ego figured out how to manipulate matter and energy around him, which he used to create his own planet as well as a humanoid form that would explore the cosmos. However, he needs to return to the home planet to recharge, and cannot be away for too long. With the help of another Celestial, he could use the seedlings he left across the galaxies to terraform into extensions of himself. The only thing that mattered in the universe was that he would become the sole life-form in existence.
All Loved Out
Though Ego had relationships with many females across the life-sustaining planets, he left them all believing he was not meant to walk among that life, for he was the only being that mattered in the universe. He would bring the offspring to his planet with the aid of Yondu Udonta, though Yondu’s betrayal when he failed to bring Peter Quill to him made them enemies.
During his pursuit of his long-lost son, Ego would come into conflict with Ayesha and the Sovereign, who were hunting the Guardians. His reunion with Peter was short-lived upon revealing his true nature and history - and an enemy of Star-Lord makes enemies with all the Guardians of the Galaxy. Nebula too would join the fight against Ego.
During his travels, Ego wooed females from many alien species and had thousands of children across the cosmos. However, he only ever shared a special connection with one woman, the Earthling Meredith Quill, who gave birth to their son, Peter. Ego truly loved Meredith, though he would also cause her death, lest that love threaten to sway him from his larger mission.