Notice the exchange that follows Jesus' telling of the Parable of the Good Samaritan:
"Which of these three, in your opinion, was neighbor to the robbers' victim?"
He answered, "The one who treated him with mercy."
Jesus said to him, "Go and do likewise."
The Samaritan treated the robbers' victim with mercy.
Too often, I think, mercy is confused with forgiveness. It's a natural enough confusion, since as sinners we experience God's mercy in large part through His forgiveness.
But mercy itself is a kind of compassion or pity, "a fellow-feeling for another's misery, which prompts us to help him if we can," as St. Augustine puts it. So while God helps us by forgiving our sins, He also helps us in countless other ways that no less than forgiving our sins manifests His mercy toward us.
How can we show mercy to each other? For starters, we can forgive each other such debts as are owed to us, as God forgives us.
Forgiving each other doesn't exhaust the ways we can be merciful -- but if we don't distinguish between mercy and forgiveness, we might think it does. We might, for example, feel we're being perfectly good Christians by forgiving someone who has wronged us, yet leaving him in a wretched state we could ease if we chose to.
We might also show no mercy at all toward someone who has not wronged us personally. After all, if "Be merciful as your Heavenly Father is merciful" means only "Be forgiving," then we have no duty toward those who have done nothing we can forgive. Perhaps that's what the priest and the Levite told each other in the Jericho inn that night.
How can we be merciful? According to St. Thomas, "a defect is always the reason for taking pity, either because one looks upon another's defect as one's own, through being united to him by love, or on account of the possibility of suffering in the same way." If we are to be merciful as the impassible Father is merciful, it must be by being united to others by love.
I'm no expert at being merciful, but I think it's safe to say you aren't perfectly united to another in love if you catch yourself asking, "Haven't I helped him enough yet?"