[xrstf]

Because using certain keys could break your JavaScript (and JSON was designed to be a subset), like

    { delete: "me" }

If you don't want to keep a list of known keywords around, just insisting on quotes is the easier way.

[EDIT: See also this YouTube video, where Douglas Crockford explains it himself: https://youtu.be/-C-JoyNuQJs?t=5m]

[yepperino]

Unquoted keywords are valid ES5.1 object properties.

    alert(JSON.stringify({delete: "me", for: 1, var: 2}));

results in:

    {"delete":"me","for":1,"var":2}

The only browser it does not work in is IE8 and earlier.

[lucian1900]

JSON significantly predates ES 5.1