I think there is both a temporary experience of happiness, like kinetic energy, and potential for happiness, like potential energy. Making yourself smile can be like converting some of that potential energy to kinetic energy. It can result in draining the potential energy.

Curmudgeon warning...

As an ex-pat living in Finland, I have always respected and appreciated that service sector workers here have in general no need to paste on a phony smile. Simple attentiveness to customers, accompanied by courteous-enough "hyvää päivää" and "kiitos", have generally been just fine.

The times are a-changing though. American-style forced smiles and performative cheerfulness are invading unevenly -- inexorably?

(I recall Monty Python's "Happy Valley" skit, where an offender is sentenced to hang by the neck until he cheers up.)