This sounds like a page out of an Emotional intelligence book or magazine, it's true happiness should not be maintained at all times as it brings about the jumpy vibe as I like to call it which makes us miss details and mismanage items which then causes sadness which in my opinion is a good thing and should be enjoyed also.

The way of enjoying might me different but it's still happiness to me, with sadness come a keen grasp of information although mostly negative ones when not controlled with positivity, but when it is you get to be a lot calmer and creative due to the new set of information, like they say the greatest motivator is a problem. Enjoy all times friend relish in both moments with a positive mindset