I have been blessed to train with a small trainload of incredible teachers.
Each, in his own way, has made me get better, some by making me think about technique; some by making me think about energy, some by inspiring me to train harder, some by making me feel like a sissy for not training hard enough.
My favorite line from a teacher, I think, was an 8th Dan from Tokyo who said "A teacher must be very severe". When he was asked, he explained that he meant that a teacher has to be very severe on himself, to force himself to keep pushing his own envelope and improving. He said that a teacher has to find a way to make training pleasurable for his students in order to keep Aikido alive, so that more teachers will emerge from the pool who are willing to be "very severe" on themselves.
From the time I heard that, I made up my mind that I had no desire to teach until I was willing to make the commitment to myself to "be severe" on myself. Priorities make that not possible now, but I hope, someday.
I truly believe this kind of thoughtfulness is the "guts" of Aikido. This is, obviously only my opinion.