Greet every person in a pleasant manner (Ethics of the Fathers 1:15).
Occasionally, when I walk into an office, the receptionist greets me rudely. Granted, I came to see someone else, and a receptionist's disposition is immaterial to me. Yet, an unpleasant reception may cast a pall.
A smile costs nothing. Greeting someone with a smile even when one does not feel like smiling is not duplicity. It is simply providing a pleasant atmosphere, such as we might do with flowers or attractive pictures.
As a rule, "How are you?" is not a question to which we expect an answer. However, when someone with whom I have some kind of relationship poses this question, I may respond, "Not all that great. Would you like to listen?" We may then spend a few minutes, in which I unburden myself and invariably begin to feel better. This favor is usually reciprocated, and we are both thus beneficiaries of free psychotherapy.
This, too, complies with the Talmudic requirement to greet a person in a pleasant manner. An exchange of feelings that can alleviate someone's emotional stress is even more pleasant than an exchange of smiles.
It takes so little effort to be a real mentsch.