I highly recommend Influence: The Psychology of Persausion. I think it has a satisfactory explanation for this phenomenon in the second chapter about Committement and Consistancy. In it, he talks about how people take cues from their actions (and imagining themselves vividly as professors would be a good way to make your mind believe you are in fact takign those actions), and that will will tend to want to be consistant with their actions and their commitments. More info here:
Robert Cialdini - Wikipedia, the free encyclopedia
The bottom link is a summary of the book, hopefully that'll help to understand this phenomenon better.