It has often been regarded as the tragedy of this mortal life that nothing lasts, all is evanescent; change is constant and unavoidable (disease is common, and mortality ends in death).
Such is the reality - and it is usually seen in terms of failure, imperfection, and general tragedy. If that was truly the case, then mortal life is a kind of failure; and such failure tends to reflect badly upon a God who is supposed to be both the creator and Good - and whose loves us.
Why would such a God make such a world?
Yet it is also possible to regard mortal life as well designed for its purpose.
This involves us regarding it as a true fundamental assumption that God is indeed the creator, who is Good and who loves us; and therefore to go on and try and understand (by empathic, intuitive identification) why such a God would deliberately make such a world as this.
We also need to understand God's purpose.
In order to avoid getting misled by hearsay and external information (from the news, from history, from third part report generally) that may be incomplete, biased or deliberately-misleading - or misinterpreted by us; we really ought to ask such questions individually, each person for himself or herself; and based on our own direct and personal experience of mortal life.
We also need to ask this question with the proper metaphysical framework of God's purpose: God's motivations for creation: what God is aiming at by creating Men.
My understanding is that God is aiming to raise Men to his own level - that is to enable Men to learn to become gods, co-creators within God's original creation. We have seen this happen with Jesus Christ; and the intent is that as many others as possible will follow this trajectory.
Such a plan depends on the will and consent of individual people; each must choose this path: this choice coming after our biological (mortal) death. It also depends on individual people learning from experience.
Therefore, this mortal life is a finite period during which individual Men may have experiences, from which they can learn what they need to be able to make an 'informed choice' in favour of accepting Jesus's offer of Life Eternal in Heaven.
From such a perspective, the changeability of mortal life is a design feature; it is necessary and it is optimal that each experience be short-lived - and then we move on to another experience. Indeed, it would negate the value of life if peple were to attain a state that did not change - since they would cease to learn, would fail to take advantage of other possibilities of learning.
But imagine if you could stay 'in love', with maximum intensity and without diminution or alteration, for decades upon end - until the moment that you were struck down by death? How much could be learned from such a life, as compared with a life in which a person experiences a wide range of changing love; and hatred, fear and loss?
This is often harsh, unpleasant, horrible; because we always lose what we have gained; solid happiness remains out of reach or slips from our grasp. We suffer.
But such negative suffering is itself a temporary state. And it serves a purpose. because without the negatives - would be really understand and appreciate the positives?
When, after mortal life, we come to make a choice for love - or to reject it - it is surely helpful to know love from both sides and in several forms? That is what the changeability of mortal life does for us.
For some people; that way, and only that way, can they be brought to a proper appreciation of the value of love, and the horribleness of its rejection and exclusion.
Only thus can they make an informed choice for or against Heaven; only thus can they really know what they are doing.