In The Merchant Of Venice, Portia suggests that Shylock be merciful to Antonio. Which reasons does she give for this?

When Shylock asks Portia on what compulsion he should be merciful, she offers the following insights:

The quality of mercy is not strain'd,
It droppeth as the gentle rain from heaven
Upon the place beneath: it is twice blest;
It blesseth him that gives and him that takes:
'Tis mightiest in the mightiest: it becomes
The throned monarch better than his crown;
His sceptre shows the force of temporal power,
The attribute to awe and majesty,
Wherein doth sit the dread and fear of kings;
But mercy is above this sceptred sway;
It is enthroned in the hearts of kings,
It is an attribute to God himself;
And earthly power doth then show likest God's
When mercy seasons justice. Therefore, Jew,
Though justice be thy plea, consider this,
That, in the course of justice, none of us
Should see salvation: we do pray for mercy;
And that same prayer doth teach us all to render
The deeds of mercy. I have spoke thus much
To mitigate the justice of thy plea;
Which if thou follow, this strict court of Venice
Must needs give sentence 'gainst the merchant there.

She mentions that mercy has a pure, undiluted quality and is as gentle as rain. Mercy is twice blessed for it blesses both the giver and the receiver. It is more powerful than the mightiest. It is a quality that is greater than the crown a king wears, for it d his great authority. Mercy is stronger even than a king's sceptre, a symbol of his command for it is enthroned in the hearts of kings. Mercy is a God-given attribute and for one to show it, is an indication that such a person has divine qualities, especially when such mercy is companioned to justice. 

Portia explains that when justice should be served none should see only redemption, for justice should be seen to be done. However, we do, once just duty has been done, we do pray for mercy. It is that prayer which also teaches us to be merciful ourselves.

Portia ends by saying that she has said what she has in order to alleviate the justice of Shylock's plea so that it may not seem so severe, but if Shylock should insist, the court must sentence Antonio. Shylock, however, stubbornly refuses to follow Portia's suggestion and seeks redemption by lawful means. His insistent and malevolent desire for revenge so ensnares him that he becomes entangled and suffers a very unfortunate outcome.