Happiness is not about what happens to you, but how you choose to respond to what happens. That's why it's called happiness not happenness—though it could be called hope-ness. You must always leave room for hope that all has happened for good cause.
Or to quote the philosopher Arthur Schopenhauer: "Life may be compared to a piece of embroidery of which, during the first half of our time, we get a sight of the right side, and during the second half, of the wrong. The wrong side is not as pretty...but it is more instructive; it shows the way in which the threads have been worked together [to make the pattern].
You feel this Artsy guy's got it pretty right. What he says reminds you of a tip your gardener friend told you..."Some plants are only meant to last for a certain season or a certain time, (said your gardener friend). If you try to make them live longer, you will be a bad gardener."
You were struck by how the same goes for people and jobs, how sometimes it seems people and jobs—and/or problems in general—are brought into your life for certain reasons, to stay around for a certain time, to teach you certain things. Of this you are certain.