It's the prettiest large city, friendly people
LH, being a native of Boston, and having lived all over the country, I DON'T rate Boston's inhabitants as the friendliest per se...that is, in public on a stranger basis. Bostonians are mind-their-own-business types who, like many in the heavily populated Northeast, approach interaction with strangers with some leeriness, cause let's face it, there are some pretty diverse personalities in those parts.
Midwesterners tend to be much friendlier on a "hi how are you' basis.
BUT one thing I will say....when you MAKE A FRIEND in Boston, it's a friend for life. I've found that Bostonians are far more inclined than everywhere else I've lived to make friends they know well 'part of the family'. Bostonians are less nosy and far more protective of your confidence in them. In Dallas, gossiping is an art form. In Boston, it's still bad manners!