I read an interesting analysis about the dynamics and system of Bleach universe a few weeks ago and it was mentioned that Ichigo's father Isshin had put it into Ichigo's mind that he had to learn FGT and use it on Aizen in order to win. He knew that using FGT would cause him to lose his powers. The reason why Isshin did that was because he wanted Ichigo to lose his power. Because if Soul Society would learn about Ichigo's rare and strange heritage, about his true powers, they would eventually see him as a threat. Isshin didn't want this to happen. He even hid himself from Gotei 13 during the war against Aizen and waited for everyone to be defeated until he would come out. He hid himself again during the Fullbring Arc. He did all this to protect his son, who is born from a shinigami and a quincy with a hollow inside.
Actually, the analysis I read was very interesting and I'd really like to share it with you guys, since it has a lot of good points to discuss, but unfortunately it's written in Turkish =D sorry..