To pray is the obvious, natural thing for a child to do — Martyn Lloyd-Jones

"Prayer is something which is expressive of the relationship between the child and the Father. Now I think that is a very important argument. You show me a man who does not pray very much and I will tell you the real problem of that man. It is that he does not know God, he does not know God as his Father. That is the trouble. The problem is not that he is not a moral man, or that he is not a good man. He can be highly moral, he may be very faithful in Christian church work, there may be nothing he is not prepared to do, but if he does not pray, I tell you that the essence of that man’s trouble is that he does not know God as his Father. For those who know God best are the ones who speak to him most of all.
There is no need to prove a thing like this—the little child always speaks to his Father. Have you not often noticed how the child of some great man talks to him freely, while another man going into his presence is nervous. Not so the child; the child speaks freely, because he knows the relationship and so he speaks to his father. And that is why the most saintly people are the ones who pray most; that is why the Lord Jesus Christ prayed more than anybody else, because he knew God in a way nobody else knew him. That, then, is the way to approach this question of prayer. The whole trouble with people who get into difficulties over prayer is that they start at the end instead of at the beginning. You do not start with the desire for answers, you start with adoration, and it is because we forget this all important matter that we tend to get into such perplexities. To pray is the obvious, natural thing for a child to do and there is nothing that expresses more eloquently or more cogently the whole relationship of man to God as prayer. That is the first thing. So, then, I think that the saints and, supremely, our Lord himself, prayed to God, primarily, not to ask for things but to assure their own hearts and to maintain their contact with God and to make certain of their contact and communion with him.
Our whole idea of prayer is false. We think of prayer only as guidance and requests. Now if you were to put that into practice in human relationships you would regard it as insulting. No, the thing the saint wants to know above everything else is that all is well between his soul and the Father. There is nothing the saint delights in more than to know God as his Father. He likes to maintain the contact and communion, to assure his heart before God and in the presence of God. The saint is in this difficult world, there are temptations from the outside and the whole world is against us, and the saint is tried—sometimes he almost despairs. So he goes to God immediately, not to ask this or that but just to make certain that all is well there, that the contact is unbroken and perfect, that he can assure his heart and know that all is well.
That is what our Lord is doing here in John 17, and that is the thing which stands out most frequently in this prayer. Our Lord is assuring his own human heart in the presence of his Father…"

— Martyn Lloyd-Jones (2000). The assurance of our salvation: Exploring the depth of Jesus' prayer for His own: Studies in John 17 (33–35). Wheaton, IL: Crossway Books.