Blessed are the merciful, for they will be shown mercy (Mt. 5:7).
When we talk about God's love, we often describe it as unconditional (Rom. 8:39). When we think about God's mercy, we celebrate it as inexhaustible (Lam. 3:22-23). Still, some are surprised when they discover that terms and conditions apply (though not necessarily what we think). It's not that we can pay God back. We never can. It's not that we will make ourselves deserving. We won't. It's not that we will eventually perfect this life. We are growing. We are moving towards perfection, but that's God in us and God won't be done until Christ returns.
So what are the terms and conditions connected to receiving God's love and mercy and forgiveness and compassion? Simply this: What we receive, we give. This is more than a simple quid pro quo, this for that, what comes around goes around philosophy of life. It's true that if you are merciful to others, they will more likely be merciful to you. That said, scripture repeatedly ties our mercy and forgiveness to God's.
If we judge, we too will be judged and in the same measure (Mt. 7:1-2). If we fail to show mercy, none will be shown to us (Jas. 2:13). If we forgive, we too will be forgiven, but conversely, to withhold forgiveness is to deprive ourselves of God's forgiveness to us (Mt. 7:14-15). I suppose at the very least, we should choose mercy so as not to deprive ourselves of that which we so desperately need. We are utterly dependent on God's forgiveness of our trespasses so we pray that we learn to forgive others (Mt. 6:12).
But hopefully our mercy does not stem from self-interest but rather overflowing gratitude. We know what it is to need it and receive it. How could we not give it as well-not out of begrudging obligation but overwhelming joy?