Explain how the Era of Good Feelings emerged and what factors led to its demise.

The "Era of Good Feelings" emerged in 1815 as a result of the disintegration of the Federalist party after the Hartford Convention and a lull in partisan politics between the Democratic-Republics and Federalists.
This era was named as such because it represented a time in which the sense of national purpose and desire for unity in America was at a high as a result of the War of 1812 and Napoleonic Wars. The country had begun to envision the federal government as having a permanent and significant role in the well-being of the nation, and, following suit, Madison authorized a protective tariff on manufactures and measures for a national bank. Optimism seemed to be overshadowing previous political conflicts.  
Despite the title, factions of the Monroe administration and the Republican Party were still experiencing the weight of divisive politics. Monroe attempted to include top prospective presidential cabinets and leaders in his own cabinet to manage factional disputes; however, amalgamation resulted in his failure to appeal to Republican solidarity, resulting in a loss of party discipline.
The Panic of 1819 and McCullock v. Maryland ultimately resurrected the political debates about state sovereignty, federal power, and loose versus strict interpretation of the Constitution. The Missouri Crisis of 1820 also contributed to this conflict, and the country saw a sudden decline in the mutual agreement that had marked the era. Thus, the modern Democratic Party was born as an answer to this division, and the Era of Good Feelings came to an end.