To Live is to Change
If Life had meant to stay the same, we wouldn't have time. We wouldn't have time as a concept, literally. We grow and change as beings - physically.
We constantly go through changes outside of us (defined by time), so it is only natural to do that with our minds. Paradoxically enough, our ego and fear try to hold on to where we are. We get comfortable in a situation that we find works for us, and if we overextend our stay in it, it is like being those annoying guests at parties that come first and go home last. Eventually, the situation will kick us out or go to sleep into a depression.
"Change" is the one true constant thing in life, and it is as sure as death and taxes. I often think that we come here as a play-doh of thoughts and emotions, and we have to make out of ourselves the most remarkable art piece of the material we got, and we likely have to remove it, as opposed to adding to ourselves. This process of "Via Negativa" (a Latin phrase meaning: by way of negation) is not just knowing what not to do but also what to remove from ourselves, to purify our character. As Michelangelo once said: "I saw the angel in the marble and carved until I set him free."