How did Defoe develop a deep understanding of the outcasts of society?

Daniel Defoe presents the case of a man with a very intriguing biography, one that certainly allowed him to understand the role of the outcast. As a very prolific author, he wrote a number of controversial political pamphlets early in his career. Through these pamphlets he was found guilty of sedition and libel and he was pilloried and spent some time in jail.
Of course, Defoe's most famous work is Robinson Crusoe (1719), one of the most widely read and translated novels in the world. It tells the story of a shipwrecked castaway who initially tries to save himself from the desert island he's stranded on but eventually makes peace with his lot in life and his separation from society. There is some suggestion that this work may be non-fiction and reflect the experiences of a family friend.
Whatever the case, Defoe was always a bit of an eccentric and an outsider, as most writers are. But Defoe was particular in this respect. Raised and educated by dissenting Protestants, he experienced all sorts of strange occurrences, not the least of which included surviving the plague and the Great Fire of London when he was young.