The period following the war 1812 was called the "era of good feelings" because:

Era of good feelings due to its one-party dominance, in fact, Democratic-Republicans were deeply divided internally and a new political system was about to be created from the old Republican-Federalist competition that had been known as the first party system. I found it off a website.

It was called the "era of good feelings" because of its reflection of a sense of national purpose and a desire for unity and had a one party dominance.