Western attitudes made imperialism possible. People of the West felt superior to the non-white people of the world. Therefore, they thought that it was proper for them to go out and colonize those people.
These same Western attitudes affected how they treated their colonies. This feeling of superiority caused them to ignore the desires of the conquered people. It also caused them to run the colonies for their own good and not for the good of the natives. This resulted in things like colonial economies that helped the imperial country much more than the colony. It also resulted in systems that did not do a good job of educating the natives or of preparing them to govern themselves at some later date.