Answer:
He was upset because Burr poisoned his political career.
Step-by-step explanation:
Back in the 1800s, Burr decided to run for President. It was a tie and it was now up to Hamilton to decide who would win. Because Burr and Hamilton, well, didn't really agree with each other a lot, Hamilton humiliated Burr by turning the public against him. Jefferson was elected for President, which upsetted Burr.
But wait, there's more!
Burr decided to run for the governor of New York, but again, Hamilton turned the public against him. In the end, Burr lost.
That was the last straw for Burr, which lead to America's most famous duel: The Burr-Hamilton duel.