The most obvious reason is that it builds rapport with students, though that's not specifically an answer rooted in speech act theory.
For a more specific response in line with Austen's theory, positivity helps listeners trust speakers, and it is difficult to meet the felicity conditions of recommending, a prime illocution among instructors, if his students don't trust the recommendations (read: instruction) they are receiving because an instructor is negatively critical, depressed, or the like.