Why are we here?
God is always in the right and is forever worthy of praise and adoration. It's tedious to attempt to prove such a thing from scripture. [Yes, I'm aware of passages of bewilderment and protest and the thrust of the book of Job and I'm aware too that pain and loss lead us to confusion and displeasure with God. But for all that, whatever we think, the Bible in its grand drift and countless passages proclaims the glory and worthiness of God.] With that clearly in mind let me say that God's glory includes his purpose to create the material creation and humanity for holy and joy-filled life and communion here. He creates out of love (see Psalm 136) and he created humanity with Jesus in mind as the ultimate image of humanity. He meant us to become like Jesus, immortal and splendid, living life to the full in righteousness (see passages like Colossians 1:15-16, Philippians 3:20-21, Ephesians 1:10, 1 John 3:3 and Romans 8:29-30).
God created us in his image that we might reflect his character, activity and purpose and he calls us to be holy, righteous and thankful. Of course he does! But is he vain? The idea that he created solely that he might have creatures to worship him narrows him. In the "land of the Trinity" the Father, Son and Holy Spirit eternally experienced holy love and joy-filled fellowship and it is out of that joy and love of life that God created, that others (creatures, sons and daughters) might have a share in life.
Philippians 2:5-8 gives us an insight into the nature of God. Worthy of worship? Forever and without reservation! Part of the reason we adore and admire him so is that he is not vain or self-absorbed! He is bent on giving us life to the full!
It isn't true that God loves us because we are here! We are here because he loves us and he loves us because he loves himself with that radiant fulness of contentment of one who is neither vain nor troubled by a self-identity crisis. In the vision of why he created us (to reflect his image) he sees his own life-filled heart reflected. It doesn't matter what we think of ourselves! It doesn't matter that we reject his reason for our existing. It doesn't matter that we cheapen and narrow ourselves. None of that changes God's desire for us.
One day—sad beyond expression—a host of us will get a glimpse of why he made us and all our cheap thrills, our self-dishonouring and our settling for some good rather than the best will come home to us with an eternal sadness.