M’nos HaLevi writes that Haman thinks that Achashverosh is referring to him because Achashverosh would only want to honor someone who already has money, so it had to be Haman.
The Iturrei Torah gives a more drush explanation. Haman focused on the fact that Achashverosh used the word “ish” (“man”). Since the Talmud (Megillah 12a) notes that Haman is called an ish, he assumed the king was referring to him. It happens to be that the same source calls the actual deserving honoree, Mordechai, an ish, as well.
Perhaps Haman also considered that the mispar katan of ish (1+1+3=5) happens to be the same as that of Haman (5+4+5=14=1+4=5).
The Sfas Emes writes that Haman was thinking that even if Achashverosh were referring to someone else, he will still do this for him later, as well.