Answer:
Because they believed in the cause of American Revolution. They believed that people have their natural rights, and that one of them is liberty. They fought in America because they believed that people in their countries they also should also upheld similar ideas and movements. I think they also did it because they believed that rights of people are above the rights of individuals. That is why they fought for Americans as for their own people.
Explanation:
Lafayette, Pulaski and many other revolutionist who fought in the American Revolution continued spreading the ideas of revolution even later. They truly believed that inequality should be removed, and that sometimes the revolution is the only solution.
