MIT top US university for study, Brown 'happiest'

Every time I read articles like this one I don't understand how they can measure the happiness of the students.

How happy are you?[0,5]

Brown has very liberal grading policies and you can even take all of your courses pass/fail.

There really isn't much of a required curriculum either.

If you are only taking classes that you actually wish to take or see the value in taking and then have less burden worrying about grades/your CPA is it really surprising that they are the happiest?