Enthusiasm is very nice when genuine but if that version isn't available then a reasonable facsimile will do.
People who demand that they must feel a particular way in order to behave in that manner can be huge drains on a team. The best colleagues have a positive demeanor even if they don't feel like it.
Individuals who believe that their feelings are the key criterion mistakenly conclude that their colleagues just have better moods. That is not so. On any given Monday morning, most meeting participants would rather be on a beach in Tahiti but they have the decency and professionalism not to show it.
This operates in all directions. The leader who blubbers about every problem he or she encounters is not doing the team any favors. The team members have their own problems and should not be unduly burdened with the leader's challenges. The reverse is also true.
To some extent, acting a part is a requirement of every job. We may choose to be sensitive cops or tough cops, activist or corporate lawyers, gregarious plumbers or quiet ones, but we have to be able to project the central role.
And that requirement exists regardless of our feelings.