Nicodemus just would not look outside of the box. Instead of thinking in spiritual terms he was only thinking of earthly things. We must be careful of only thinking of things of this earth. When we restrict our thinking to only what we know and understand we build a box around ourselves.
Jesus is challenging us to think outside our box. Through baptism we are brought back into a right relationship with God. God is always with us, but he grants us free will and we can make our own choice. Can we step outside of the box to be taken up into Christ?
The difficulty of this is that we are between two worlds. We live in this world physically but are children of God.
A good example is our understanding of the Eucharist. We need food, bread and wine, to live. What we eat becomes part of us. We need spiritual food, the body and blood of Christ. When we eat of this spiritual food, we become one with God in the Body of Christ.
May the Lord bless you in the name of the Father and of the Son and of the Holy Spirit. Amen