Why did Fredrick Douglass become an abolitionist?

The simple answer to this question is that Frederick Douglass spent the first twenty-three years of his life as a slave. His experiences as a slave inspired within him a deep, abiding hatred for the institution that had kept him and millions others like him in a state of subjection. Unlike many other slaves, however, Frederick was able to read and write. Indeed, he saw literacy as providing a freedom of its own. He had a keen intelligence and was acutely aware of the world around him. It wasn't enough for him to be free; he needed to go out and let the world know of the evils of slavery and make freedom a possibility for others.
Inspired by William Lloyd Garrison, famed abolitionist and editor of The Liberator newspaper, he became an active abolitionist himself, a tireless campaigner for the cause. He made countless speeches, wrote numerous books and articles—including his famous autobiography—and campaigned for African American troops to be allowed to fight in the Civil War.