The major factor involved with westward expansion was violent confrontation due to polar opposite forms of culture and government. Any oppositions from the native americans was quickly shut down by the U.S. Government.
However without western expansion our nation would be 2/3 of the size it is today.
Bad effects:
The buffalo population was depleted, the land went from untouched to developed for people, and all the grass was grazed, the water contaminated
Good effects:
People wanted to go west because they sought the opportunity to own land and make money. And the expansion of the railroad