Aizen was in love with Kirio Hikifune. She was the reason Aizen joined the Gotei 13. He trained and trained to become as strong a shinigami as he could. He become strong enough and was finally able to become eligible to join the gotei 13. He wished to get into the 12th division to serve under Hikifune, but was instead put into the 5th division.
Everything went fine for awhile. Hikifune seemed to be ignorant of Aizen's love of her, but he did not give up. But then, Hikifune fell in love with the 11th Division Captain, Isshin Kenpachi. Aizen became extremely jealous. His jealousy became hate. He wanted to kill Isshin Kenpachi, but couldn't find a way to get away with it, even with his complete hypnosis, so, he found a way to separate his Bankai's power(because no one knew he had bankai), leaving Aizen with only his shikai, so that his separated bankai could kill Isshin. Aizen traveled deep within soul society, to District 80, and separated his bankai from his zanpaktou and gave it human form.
The separating of the bankai portion of his zanpaktou left this newly formed being without any memories. Aizen kept his distance from the bankai, leading it towards the path that it was created for. The bankai named itself Zaraki, and strived to be known as the strongest, something the current 11th captain, Isshin Kenpachi often gloated of being.
Meanwhile, during a mission that involved going into the human world, Isshin had fallen deeply in love with a human, and wanted to leave Soul Society to live with her, but that was not allowed, and he still did care for Hikifune. One day, he found out that Hikifune was given an opportunity to become a part of the Zero Squad, but she refused because of her love for Isshin. Isshin believed her joining Zero Squad would be the best decision she could make, so he tried to figure out a way get her to join it.
Zaraki had made his way to seretei(sp?), making his claims he was the strongest and wanted to prove it. Isshin had found a way out of his dilemma. He Made a challenge to Zaraki, saying they would fight, and the winner would get to keep the title of Kenpachi.
During the fight, Isshin faked his death, using this chance to escape to the real world. Aizen, aware that Isshin faked his death, kept track of him, but did not care because now he was not with Hikifune.
What Aizen did not foresee, however, was that now that Isshin was out of Hikifune's life, she now had nothing keeping her from joining the Zero Squad, which she did. Aizens heart broke. He fell into depression. He tried and tried to become stronger so he could get promoted to Zero Squad, but he was at his limit. A limit self imposed psychologically by Aizen himself, because deep down, he knew, that even if he did get into Zero Squad, Hikifune would only break his heart again.
He tried his best to become even stronger, but to no avail. Then one day, he found out about an invention made by the new 12th division captain, Urahara. The invention was the Hogyoku, an item that breaks limits. Aizen set up Urahara and a large majority of the Gotei 13, who had often made fun of him and his crush on Hikifune. He hollowfied them all, and made it look like it was Urahara's doing. As planned, Urahara was blamed, but he managed to escape with the Hogyoku before Aizen could take it.
During the next 50 years, Aizen created several experimental hollow/shinigami hybrids. The Shinigami side was used to give emotions and "heart", while the hollow served as a barrier and restriction to the power of the heart. Aizen's obsession consumed him. He was no longer of sane mind. He had become so consumed with breaking the limits of his heart, he had forgotten why he was doing so. He could only remember that he wanted to get into The kings dimension. Soon the events of the Soul Society arc occur, and Aizen obtains the Hogyoku.
Aizen retreats to Hueco Mundo, where he continued his hollow/shinigami hybrid experiments. He created dozens of Arrancar trying to create a hollow with a "heart", for if he can create a hollow with a "heart" surely he could break the limits on his own heart. His experiments do not meet his expectations, but he succeeds in unlocking the true power of the Hogyoku. Believing all his experimental arrancar were failures, he disposes of them while using them to distract the Gotei 13, which brings us to the current chapter of Bleach.
Aizen is fighting the man he hates the very most, though he know longer even remembers why. He reveals to Isshin the power of the Hogyoku, and destroys the limit Aizen had once set upon himself. Suddenly he remembers why he had done everything he had done. He is overcome with grief once he realizes that Hikifune will not ever love him, especially not now that he has done all that he has done. He attempts to repent when Gin destroys the pillars that brings the real town back. Aizen tries to get Gin to stop, but Gin slices up Aizen, leaving him barely alive.
Gin begins the creation of the key. Ichigo and Isshin try to stop Gin, but it is too late. The key is created and a portal opens up to take Gin. Isshin, Ichigo and Aizen are also taken into the portal and transported to the Kings Dimension. Hikifune stands in front of them. Aizen, who's barely managing to stay alive, stretches out his hand towards Hikifune and tries to say "I love you" just before Hikifune stabs her sword right into Aizen's neck. Hikifune then tells Aizen that he has been a great pawn, and tells him thanks for opening the gateway between the Kings dimension and the real world. She then says that now her and her son Gin can now rule all SS, kings dimension and the human world.
Bam. Suck it, Kubo!