You always have a choice of emotional response to life.
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.