When a person leaves a school or a job for another school or job, he should keep in mind what he is grateful for. Even if he feels he is benefiting more from his new school or new job, he should remember what he gained in the past from the previous places.
There are many reasons why a person might have been dissatisfied at a previous place where he was. Nevertheless, he still gained and benefited from the people of that place. It's easy for many people to remember the negative aspects of previous places, especially if a person left on not such friendly terms. They might want to throw verbal stones at the prior school or job. Our debt of gratitude tells us not only not to speak against that place, but to say or do things that are an expression of gratitude.
Look at the previous place with a good eye. When you speak about it, begin your sentences with, "I am grateful to them for…" When you do so, you are expressing your own positive character.
(From Rabbi Zelig Pliskin's book: THANK YOU! Gratitude: Formulas, Stories, and Insights: Artscroll Publishers)