Why is the quality of mercy "twice blessed"?

Portia, disguised as a lawyer, gives a speech about mercy in The Merchant of Venice in which she states that the quality of mercy is twice blessed: She says:

It is twice blessed: It blesseth him that gives and him that takes

She notes that the recipient of mercy is blessed. This is understandable. If you are expecting the death sentence, for example, and the state shows mercy and sets you free, you would most likely feel blessed.
However, Portia says that the person who grants mercy is also blessed. This is because mercy is a characteristic of God, and the person who grants it is acting with the spirit and majesty of God. Thus mercy is twice blessed: it blesses the one who gets the merciful treatment and the one who grants it.
Portia goes on to say that the more powerful a person is, the more mercy stands out in him. It stands out, for example, in a king, because a king has tremendous power and could easily behave cruelly. Nevertheless, we admire a king who is merciful because his mercy shows strength of character and empathy: all of us should recognize that when it comes to divine judgment we are in need of mercy rather than justice. We should therefore practice the mercy we hope to receive from God.
This is a very Christian speech, yet it appeals to Shylock's vanity in implicitly comparing him to a king as Portia tries to persuade him to be merciful.