Mercy is so, so great. An undeserving Jew begs G-d for mercy because he knows that his sins are so terrible and his mitzvahs so few. He feels that he deserves punishment, G-d forbid. Mercy comes just for this purpose. Even though someone might not deserve to be forgiven, Hashem extends His leniency and forgives that Jew.
But why then do we say that Hashem is Merciful and extends kindness? Mercy seems so much greater than kindness. Doesn't mercy already include kindness? In some ways kindness is even greater than mercy. Mercy comes when we sincerely request it. Kindness comes without our even having to ask. Such is the loving kindness of our Father Who has created us for His own glory.
May all of our prayers be accepted in Heaven, and may our lives on earth be both physically and spiritually successful.