Jim Starlin had a story to tell and it only took eighteen years to do it.
In 1973, in Iron Man #55, Jim Starlin debuted Thanos & Drax the Destroyer, two characters he thought up in psychology class. What started as a throwaway story blossomed into one of the longest running sagas in Marvel history. A lover of Death incarnate, Thanos was in search of power, enough to wipe out all life in the universe as a gift to his one true love. He would come into conflict with many of Marvel cosmic heroes including his father and brother, Mentor and Starfox; the guardian of the universe, Captain Mar-vell; Drax, who was created by Mentor with the body of Moondragon's father to kill Thanos; the Avengers, the Thing and even Spider-Man.
But it was the Jack Kirby created, Roy Thomas revamped Adam Warlock who would become Thanos' greatest threat. A genetically created "perfect man", Warlock spent so time defending Counter-Earth before journeying out to the stars in Starlin's "The Magus Saga" which ran through Strange Tales #178-181(1975) and in Warlock #9-15(1975-6). While Magus, an evil, future version of Adam Warlock, was the main villain of that storyline, Thanos gets involved to help Warlock defeat Magus, who Thanos believes is a bigger threat to his own plans.
With Magus defeated, Thanos sets his own plan into motion, assembling the Soul Gems to use their combined power to control the universe. He wrests the Gems from the Elders and is able to take the Soul Gem from Adam Warlock's corpse, a death he engineered to get rid of the Magus. Thanos is almost successful, until Spider-Man frees the spirit of Adam Warlock from the Soul Gem, who imprisons Thanos in stone, as told in Avengers Annual #7 and Marvel Two-in-One Annual #2(1977).
Not content with the story so far, Starlin 'resurrected' Thanos in Silver Surfer vol. 3 #34(1990) and sent him again in search of the Infinity Gems in Thanos Quest #1-2. He succeeds, places the Gems onto his left gauntlet and brings us to Infinity Gauntlet, the 1991 Marvel-wide crossover that gave us the definitive story of Thanos and his mad quest, which saw the resurrection of Adam Warlock and his friends, Gamorra and Pip the Troll, the death of half the universe, and the last stand of Captain America(makes more sense when you read it).
Say what you will about Jim Starlin and his work(and I have), but the man has passion and the drive to get a story in print.