I just came across an excellent post by Henry Karlson, explaining why this is
The freedom we need for true happiness requires us first to deny ourselves; being rooted in what we are now leads only to a dead end. . . . If we struggle against God’s grace, against his providence, if we struggle to remain as we are now, we will eventually find ourselves getting what we struggle for: ourselves, entirely closed to anything but ourselves. . . .
If we want self-fulfillment, we must deny ourselves. We must open ourselves to God and his infinite freedom, so that we can then find our place in the world. Once we know it, then we can act upon it. . . .
One might want to understand deeper how this overcoming of the self leads to true freedom.
This is perhaps best done by analogy, which will lead us to our final remarks. Consider what happens when two people, with one being infinitely strong, wise and good, and the other being weak, of indeterminate wisdom, and of indeterminate moral character are tied together by a fifty mile-long, unbreakable rope. They are at a great distance from each other, perhaps ten or more miles apart. The strong one, in their wisdom, is moving forward, to a place they know is good and will be beneficial to both. As the strong person progresses, everything is getting better and better, leading to greater and greater happiness.
The other, however, has a choice. They can struggle against the one who is moving forward like that, and try to remain where they are (or even to go in the opposite direction). If they move in the same general direction, both are free, and both are capable of moving as they wish. If the weak one tries to go in a direction contrary to the strong person, they will find themselves eventually at the end of the rope; if they struggle, they will be dragged against their will, and the rope will have become like shackles unto them. What should be guiding them to happiness is thing of suffering. On the other hand, if they see the direction the rope is trying to take them, and they decide to follow it, the rope will no longer become tight, but will be a guide, telling them a direction they can travel to obtain their own happiness.
They are not necessarily forced to travel in the exact same way as their guide, there is a large amount of leeway given by the rope. But it is guaranteed, as long as they follow the direction of the one who is at the other end of the rope, they will find themselves more and more happy. Clearly, as they see what is going on, the further they will go; they might enjoy a path which the other has not taken, to see other goods the other has not seen, but they will find themselves constantly finding new and greater forms of happiness.
Now, if the strong person can do this infinitely, leading to ever greater and greater joys and happiness, we have our analogy to God’s providence and how it is guiding us. We can struggle against it, go against it, and find ourselves ultimately trapped in a pain of our own choosing, at the end of our rope; or we can follow it, move slowly forward, and find ourselves guided to great joy.
Our faith is manifested in our willingness to follow where the rope would lead, even though we might not see the person leading us. God moves at such a pace that he gives us the ability to catch up with him, to see his movements, and to follow near him, which is exactly what happens in theosis, as the Apostle John tells us, “Beloved, we are God’s children now; it does not yet appear what we shall be, but we know that when he appears we shall be like him, for we shall see him as he is” (1John 3:2 RSV). Even then, we will have not found ourselves at journey’s end, but at its beginning. For, as C.S. Lewis points out, there will always be “further and further in” for us to journey. And we shall not do it alone. Part of the joy and happiness will be in how we share that joy and happiness with others — a joy shared is infinitely greater than a joy kept to oneself.
. . . This leads us back to the need for self-denial. We must be willing to follow Christ to the cross:
And he called to him the multitude with his disciples, and said to them, “If any man would come after me, let him deny himself and take up his cross and follow me. For whoever would save his life will lose it; and whoever loses his life for my sake and the gospel’s will save it. For what does it profit a man, to gain the whole world and forfeit his life? For what can a man give in return for his life? For whoever is ashamed of me and of my words in this adulterous and sinful generation, of him will the Son of man also be ashamed, when he comes in the glory of his Father with the holy angels.” (Mark 8:34 – 38 RSV).
Once we have died to the self, we can experience the glory of the resurrection.