What a wonderful question: Why is God relational?
Knowing God is everything: John 17:3 and Phil. 3:10.
God makes Himself know in relationship. Creation itself, is God making Himself known. Consider the trinity, we say the triune nature of God. Talk about relational! The three-in-one Truth that 3 IS 1. the Truth of John 10:30, the abiding Truth of John 14: 20, and John 17: 21; and the fellowship we have, one with another, in this mystery of the Body of Christ..
Did you know that the three primary meanings of LOGOS are reason, word and ratio. Not that's a relationship. [Eventually The Blue Letter Bible and Thayer's Lexicon get around to that fact. [u]Can you tell me what ONE English word carries ALL that those THREE words contain[/u]? Translators of John 1: 1 are faced with picking one word for logos. Most choose Word. So do I, for one over-arching reason: In the beginning God created.....He said. But oh, the relationship of reason, word, and ratio in the Truth of John 1:1.
Your question, Why is God relational? touches on the nature of ratio. [u]Ratio IS the relationship between two things[/u]. Expressed mathematically in numbers, the ratio of 1 to 2 is written 1/2. This mathematical expression 1/2 is different from 1 and different from 2. Put into words, ½ is one [u][b]is to[/u][/b] two or one separated/divided into two. I hope you can see creation in this. Do you see that when 1 is in relationship to anything, creation begins.
Here is some more creation:
Fill in the blank: one is to two, as two is to ____ (what). Put into numbers, this question is Fill in the ?: ½ = 2/? When you answer that question (solve that mathematical equation) you again have something NEW. In terms of numbers, you have 4.
Ratio, RELATIONSHIP, is very rich, it is a very rich word, it is a very rich LOGOS (Eph 2: 7, Eph. 3:8, Col. 1:27). It is even possible to see the Trinity in the word logos (reason, word, ratio: God the Father, God the Son, God the Holy Spirit. We are born of the Spirit of God, born again in (into that) real, personal, living, RELATIONSHIP.
OK. I probably should have left this post in its short form: [b][u]God makes Himself know in relationship. Creation itself, is God making Himself known[/b][/u]. We all know the Truth of Romans 1: 20: God makes Himself know in creation. And we all know the necessity of the new-birth that restores/gives RELATIONSHIP with God. Holy riches. I could not say enough in praise of God and thanksgiving for God's relational nature.
The whole of God is everywhere. Even in mathematics :-) which I once did not like very much. But He re-news our mind.