The question came up today at our parish preparation for First Reconciliation. It seemed to stump at least one student. Perhaps she didn't know the answer, or perhaps she didn't know the reason behind the answer.
At long last she ventured, "Because we can't get to heaven otherwise?"
The answer took me a little by surprise; I guess I was expecting something like, "Because Jesus told us to" or "Because God wants us to" or "We have to forgive if we want to be forgiven."
Of course, she's right; it's just not what I expected to come out of a second-grader's mouth.
And why is that? Why did Jesus teach us to pray, "Forgive us our trespasses as we forgive those who trespass against us", why does God want us to forgive others--to the point that we can't get to heaven otherwise?
Many of us can see that would be hypocritical to ask for forgiveness from God while refusing to grant it to one of his creatures.
It came to me today, when trying to explain this, that it also has to do with the nature of heaven. If heaven is the fulfillment of our existence in being united with God, we are also united to everyone else who's united to God. If he is our Father, how can we be at enmity with one of his children? There can't be any discord in the heavenly home.
Another image: no sin can enter heaven. Those who cling to serious sin, who choose the sin over God and refuse to repent of it, go with it to the trash heap (Gehenna). That's where sin belongs, and if we won't let go of it, that's where we'll go too.
But if we want to go to heaven and seek God's forgiveness, then he will remove the sin from our souls before we enter its gates. To seek that forgiveness but refuse to give it to our brother is like trying to leave our own sin at the gate, while trying to bring his sin in and paste it back on him ... or trying to keep him from going in at all.
It just won't work.