Why do Boston bars close so early? Something of the old Puritan ethic of the Massachusetts Bay Colony lingers in the so-called "blue laws" that prohibit sales of alcoholic drinks at bars and restaurants after 1 AM on weekdays and 2 AM on weekends. The state remains of two minds when it comes to social leniency. The first state to legalize gay marriage was also one of the last states to allow liquor sales on Sunday. (Both became legal in 2004.)
Historians surmise that the origin of the blue laws goes back to colonial times when special laws were actually written on blue paper. In 17th-century Boston, it was forbidden to walk on the street on Sunday, or to sing, dance, fiddle, pipe, or use a musical instrument at night. Most of these laws have been repealed (though it's still technically illegal to sit on the grass on Boston Common without a proclamation from the mayor). But periodic attempts to push back closing time still meet with heavy opposition from conservative neighborhood groups.
Late-night revelers party on in other ways. Asking for "cold tea" at certain Chinatown restaurants might get you a beer, and at certain Irish bars around town, the lights are off but somebody's home. A modern trend has been to form "private clubs" such as Rise, on Stuart Street, where members pay yearly fees for the privilege of partying (although not drinking) all night. Thankfully, it's no longer illegal to dance until dawn.