A friend from college became quite wealthy in the debt collection business. Like others said - high volume. I never heard she was particularly unpleasant. Persistent yes, hostile no. Just get a bunch of judgments (often default) and attach and garnish what you can. Pretty much everyone in the chain just gets paid on percentages.
I did my share of collections and bankruptcy work on behalf of creditors. Not a really happy business, but can be quite profitable if done correctly.