Many faculty will periodically get warm, thankful emails or cards from former undergraduate students who genuinely appreciated a class, are grateful for a letter of recommendation, or see the value of a concept or theory only years later. I know that I am genuinely flattered and encouraged by these types of communications, and I’m sure many other faculty are, as well.
As a faculty member going into my 20th year, one thing I can definitively say is that you have to remind yourself to believe in the far-reaching effects of your efforts in the classroom. There are few metrics that allow you to put a precise “value” on teaching, unlike the number of downloads something you’ve published has, number of citations you’ve gathered through scholarship, or the “impact factor” of a journal you’ve published in.
Fellow professors: if you feel so moved, sometime during your busy work week, take a few moments and be kind to your body and mind, and remind yourself to believe in the far-reaching effects of your teaching!