Why does God want us to be pure?
That we may be like him.
So that we may love Him the way we should, because He is pure. No one can see God unless he dies (dies to himself, killing his sins). Because nothing can enter the Kingdom which is sin, so the purer the better.
Think of Fr. Barron when he says: A man says to his wife: I love you! You are beautiful, you interesting, you are my life! Yes i love you. Ninety % of the time. Well, no, it is not the proper way.
God wants for us the best way. I know it is hard at times, but since we believe we are made in his image, and lost it, we have been gracefully given another chance to be identical to Him.
This is what Jesus said to St. Margaret Mary. (She was the saint to whom Jesus manifested his Sacred Heart.)
“One day, on account of some fault which I had committed, my Divine Master gave me the following lesson. Learn, He said, that I am a Holy Master and One that teaches holiness, I am pure and cannot endure the slightest stain. Therefore thou must act with simplicity of heart and with an upright and pure intention in My Presence.”
We shall not enter heaven until we are pure. Any lack of chastity is a spiritual wound which hinders one’s capacity to love persons in a perfect way, and it also reflects inordinate self-love which does the same. It soils our white baptismal garments.
There is no holiness without chastity.
My favorite analogy is that God is like the sun and we are like a glass house. If the glass is dirty then the light will not enter us and we will be all dark inside. It is only by keeping the house clean that we will have the light fill us.