Why did westward expansion appeal to Americans?

The Americasn were interested in expansion northward because they believed that the Britsh North American contient should belong to the US, following their belief of Manifest destiny. Also, after the American Civil war fought in the 1860's between the Northern and Sounthern states, the southern used Canadian territory to attack the Northern. So, at the end of the Civil War in 1865, the Northern accused the British colonies of aiding the Southern states. This made the relationship between the Us and Britsh North America uneasy, and so the British North American contient was afraid that the Us was going to expand Northward and take their terrritory in revenge.