Moab is trembling, worried that the Israelites are coming for them. (Do you ever worry needlessly?) They seek to hire Balaam to curse the Jews. G-d gives Balaam three no's: don't go, don't curse them, and, no-thank-you, they don't need your blessings.
What's so terrible about a blessing from Balaam? And in the end, doesn't Balaam in fact bless the people?