Love can lead people to forgive and/or forget past mistakes or crimes. Frankly, if I didn't love humans, I would be unable to show them any mercy for the things they've done and still do. If you love someone, you will naturally be merciful to them or they may cause you to be merciful towards others. They don't say love conquers all for no reason. It causes anger to slip away and coaxes forgiveness out of the cruelest of individuals. It is certainly not something to be underestimated.
Love is a strong emotion-one that can beat many others. Love can cause people to have mercy, but having mercy doesn't always cause love. Love unites us together in teams of friendship, intimacy, respect, and affection. Love can beat hate, but first you have to love to have mercy. Love truly can, but doesn't always, trump all.
Mercy is measured in actions. It is something that has a tangible effect and is not something that is left to the imagination to see how it plays out. There are no prerequisites to merciful behavior other than personal choice and it is not grounded in anything but personal mantra making it more developed and dependable as well.
Love is too specific to get a useful reaction. Unless you know someone, you can't love them. Many people throw around the word "love" like it has no meaning. You may say "I love that actor." or "I love that musician." but seeing that you only know them through their profession means you don't know them at all. For all you know, they are the worst piece of human scum that existed. This limits any level of action to those you know personally. Personally, I can show mercy to someone I don't love, I may even hate the person and show them mercy. It all depends on if they deserve it.
Love encourages mercy, yes, but mercy does not need love. To forgive someone or show them mercy is not an entirely selfless act, many times we do it for ourselves. I can show even the most despicable person in the world mercy just by realizing that to not be merciful would be to commit an act I could never forgive myself for. I can hate the person I show mercy to, hate them for the rest of my life and never forgive them, but that doesn't mean I need vengeance.