Happiness happens when a moment cannot be altered. But whether we think a moment should be altered depends on the context we build. Or, more specifically, on our interpretation of the current situation in the context of our interpretation of the past and the future.
In other words, when you feel unhappy, it’s not because of the circumstances (alone) but more so because of the fact that the current moment is not on your imaginary line between the past and the future. Very often our expectation of this line that it is pretty steep.
While that might work from time to time, usually this gets saturated very quickly. So, if you’re unhappy often, try and think about your happiness curve and whether it is on a realistic path.