As a teacher, a post like the one below can really make my day:
This is the kind of impact I hope to make on the world, and when I (only occasionally) receive a note like this, I can hope to believe that I am making the kind of impact that I wish to make.
An acquaintance of mine this week referred to the work I do as "working in an ivory tower" or words to that effect. A relative of mine several months ago similarly referred to my writing as "batting theories around" or some equivalently flippant dismissal. It is easy to take criticisms like these very seriously. It is very hard, in addition, not to take them personally (they are, after all, meant personally).
In a world where one's sphere of impact can oftentimes be very small – even for people who are allegedly very powerful – a note like this from a former student is a great reason to keep teaching, keep theorizing, keep criticizing, keep writing, keep thinking. We're talking about people's lives.