Esther 6:6

"When Haman entered, the king asked him, 'What should be done for the man the king delights to honor?' Now Haman thought to himself, 'Who is there that the king would rather honor than me?'" NIV translation

So the king asked Haman for advice on how to reward Mordecai.

Haman, in his prideful heart assumed that the king was talking about rewarding himself.

Esther 6:7 - 9

"So he answered the king, 'For the man the king delights to honor, have them bring a royal robe the king has worn and a horse the king has ridden, one with a royal crest placed on its head. Then let the robe and horse be entrusted to one of the king's most noble princes. Let them robe the man the king delights to honor, and lead him on the horse through the city streets, proclaiming before him, 'This is what is done for the man the king delights to honor!'" NIV translation

Haman is thinking of the great parade and honor that is ahead for him as he answers the king's question. He suggests a royal parade through the streets of the city to let everyone know that this is the king's top man.

Esther 6:10

"'Go at once,' the king commanded Haman. 'Get the robe and the horse and do just as you have suggested for Mordecai the Jew, who sits at the king's gate. Do not neglect anything you have recommended.'" NIV translation

Imagine how Haman's jaw must have dropped when the honor that he suggested for himself is to be given to the one that he hates the most. Mordeaci was a Jew (one of God's chose people) and God takes care of His own. God must have a sense of humor as only He could come up with something like this. God is the same today as He was at this time and so, if you are His people and others are messing with you, humble yourself and wait on the Lord because he will honor you (like Mordecai).