It seems reasonable to at least consider that many of those getting the vaccine were wanting to go back to a "normal" life, trusted their leaders/institutions, and were under the impression that getting the vaccine (especially the 2nd one to be "fully" vaccinated) would enable them to do so.

One could argue that it's not necessarily the vaccine itself that's responsible for the upbeat mood, but rather the result of the vaccine-taker's belief of their newfound safety, their sense of heroicism ("doing the right thing & saving lives"), and their now reestablished access to a pre-covid social life (ex. allowed to move freely without restrictions, see whoever they want, not have their job/education be under threat, etc).

As such, this finding may in reality highlight the power of government control/propaganda and the human yearning for freedom rather than showing any direct health benefits of COVID-19 vaccines. Remember the classic saying: "correlation does not equal causation"; while it may be an exciting prospect to have stumbled onto a miracle drug, the reality is that basic common sense points to very glaring and obvious established relationships (which we already know to be true) to explain the outcomes presented in this article/study...