Why does God not forgive us if we are not forgiving to others?
Jesus came to generously bring forgiveness for sins for all that would ask. He brings restoration of relationship with God. And he asks us to love as God is loving.
Problem
In the middle of all this generosity and love, Jesus says (Matthew 6:14-15) that we are required to also forgive others. In fact, if we are unforgiving to others, God will not forgive us! How can God be loving, but hard-edged on this topic at the same time? Is this just a quirky hot-button issue for God or maybe capriciousness?
Explanation
Jesus came to establish a spiritual Kingdom based on all its citizens sharing his value system. His value system of sacrificial love for the benefit of others is his most defining characteristic. The most clear expression of his values was his willing death on the cross for us.
Without God, our native value system and our value system of choice is selfish love for the benefit of me - the value system of the world. This one is in conflict with the value system of God, therefore we cannot have relationship with him. We have no common values on which to base the relationship. The only possible result of this is permanent separation from God.
God's solution is to make a new Kingdom with humans under authority of a new King, and at the same time to transform them to have the value system of God. The first enables handling issues of the past, and the second enables relationship going forward.
When we become a citizen of God's Kingdom, Jesus is our king, especially with regard to relationship with God. Therefore, in that authority/responsibility relationship he has the right to substitute himself for us with regard to the consequences of ungodly values and sin. On the cross, Jesus lived his values and took the consequences of our previous ungodly value system by experiencing death of relationship with God the Father.
Adopting the value system of God is the only way to enter his Kingdom and to have a relationship with him. Forgiveness is the most fundamental evidence of the value system of God in us. Therefore, if we do not forgive each other, that is evidence we never were in God's Kingdom, or have put ourselves out of God's Kingdom. If we are not in Jesus' Kingdom, then he cannot substitute himself for us, therefore he is unable to extend forgiveness to us and we stand responsible for ourselves before God. (See also forgiveness process)
The value system perspective used here is based on ideas from the work of Darren Twa.