A new broom sweeps better. Does this apply to politics? A new political regime ought to make voters feel good while the goodwill lasts. It's simple: we don't know much about a new regime and can, therefore give it the benefit of doubt.
The voters feel good when there is a change. They exhale with great expectations. That is often whipped up by political campaigns. By the same token, new marriages are expected to be happier, the couple knows less about the realities of marriage.