Why is Robert Downey Jr. (quite possibly, the finest thespian alive) a Junior? Obviously, it's because of his father, Robert Downey Sr. (Perhaps not so obviously? Read along...)
Downey Sr. had a lengthy career in the entertainment industry. He is mostly known for directing "Putney Swope", a cult 1969 satire that took on the advertising industry. Later on, he switched to directing low-brow comedies designed to push his son to the limelight. (It worked?)
And, strangely enough (especially because he is Jewish), Robert Downey Senior is a Junior himself... Robert Elias Jr., his name at birth. He switched his last name to match his stepfather's. So, in theory, Robert Downey Jr. is really Robert Elias III.
See, this is why most Jews don't name their kids after themselves... all these suffixes just get too confusing!