Why do the the Father and Son love each other?

What is it exactly that the Father loves about the Son and the Son about the Father?

The Father is the initiator, the first cause of all things, which makes Him the most valuable and worthy of highest regard, adoration, and praise over all beings. Nothing comes close to the infinite worth of God. All life comes from Him, all love, all beauty, all power, all knowledge - everything good and right is from God.

The Son is the means by which the Father reveals himself and acts out these qualities toward His creation – creation being everything that ¹comes from God that is not God. Without Christ, we would not know or see God as He truly is. To see Christ is the see the Father. And if we wish to see God we must see Christ. Christ “… is the radiance of God’s glory and the exact representation (copy or engraving) of his being...” (Heb 1:3a). The Father delights in being known so others might find life and joy in Him and therefore He delights in ²Christ making Him known.

Because the Son willingly and delightfully agreed(s) to be the means by which the Father reveals Himself -- in and through His incarnation, death, and resurrection -- the Father has utmost regard, adoration, delight, and praise for the Son. Because the Father initiates all of this, the Son has the utmost regard, adoration, delight, and praise for the Father.

And where does the Spirit fit into this triumvirate (threefold) relational dynamic? The Spirit is God or to say it as the bible does, God is Spirit. The life, love, Spirit of God is the essence of God as the ultimate relational being. The Spirit ties, units, and holds (or binds if you will) the triune community together. The love of the Father for the Son and the Son for the Father occurs in, by, and through the Spirit. The Spirit is the infinite ³love and delight of this relationship between the Father and Son as well as the means by which this love and delight occurs, is revealed, overflows, and is poured out on others. Without the Spirit, there would be no God or love between the Father and Son. Without love and delight between the Father and Son, there is no Spirit, no God. 

For an extended discussion on the Spirit, click here. 

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*Christ also comes forth from God as the ¹only begotten of the Father. But this is not something that occurs at a point in time. The Son has always been - i.e. He is eternally begotten (He eternally issues forth)… is and will always be the only eternally begotten of the Father - i.e. the Son is God and the only eternally begotten being who is. Though He is not creation, he is begotten i.e. as the creation issues forth from God so does Christ but without a beginning. Christ has always (eternally) issued forth from the Father. The creation issues forth from the Father also but at a point in time and also issues forth through the Son. Without the Son, there would be no creation. Neither would there be a direct and personal revelation of God.

¹ “only begotten” indicates his begetting is unique to all other beings or things that come from (are begotten of) the Father. That uniqueness of the Son’s begetting is that it is an eternal begetting, making the Son equal to the Father, not equal to creation, and distinct from creation as well. He is the means by which creation came to be.

Christ is begotten and eternal at the same time. This is significant since begetting implies a beginning. There is no beginning with Christ. He is eternal and has always been. 

This is why he is the only begotten Son. He is one of a kind. He is the God/man and the connection between God and man. He holds the Father's hand in one of his hands and our hand in the other, if you will. God and man unite by Him, through Him, and in Him. For these reasons also, the Father loves and delights in His Son. 

For a more extended discussion on "only begotten" click here. 

**The Father also delights in you and I showing forth and making Him known to others. We too are image-bearers of the Father but created, not eternal as the Son is (though we are everlasting). So we are like the Son - but without all the "Omni's" - omnipresent, omniscient etc... at least in our current state (we don't know what we shall be but when we see him we will be like him - which is why Christ is called our elder brother and the first fruits of all creation). 

Since we are now righteous in Christ and to the same degree as Christ - i.e. perfectly righteous - he views and addresses us only as sons and daughters of God, wholly cherished, only loved, never rejected or forsaken.

***We could also characterize this love and delight as passionate. It is so intense it issues forth as a distinct being who is the Spirit. 

Passion originally referred to the intense emotions that occur in suffering, hence the “passion of Christ.” I am using passion in a positive sense. But the intense and passionate love the Son has for the Father and the Father for the Son and the love they both have for us, His image-bearers, compelled Christ to take on the suffering He endured to restore us back to the Father. 

This delight/love/passion occurs as the Father and Son behold the beauty of the other. This has been referred to by past theologians (Jonathan Edwards particularly) as the “beatific vision.” This is the delight the Father and Son have in and for each other as they behold the beauty of the other. It is also the delight God calls us to participate in once we are restored to the Father in and through Christ. For a further discussion on beatific vision click here.

Other posts related to the beatific vision 

The importance and necessity of the Trinity 

The essence of God's life 

Thoughts on the Trinity 

Overview of Jonathan Edwards theology.