I go for a run most days and my watch asks me how I feel afterward. I don't answer, because I really have no idea whether my "Good" answer one day corresponds to a "Good" answer on another day. Often, it probably doesn't. In my view, talking about subjective well-being is anything but simple.
It’s subjective because it cannot be universally and independently measured.
It’s not possible to measure the happiness of two individuals such that I can compare them against each other quantitatively and define who of the two is happier.
I can do that, however, with basically every physical measurement.