Why is Zaroff glad that it is Rainsford who has come to the island in "The Most Dangerous Game"?

Zaroff is glad that it is Rainsford who has come to the island because he knows that Rainsford is a "celebrated hunter".  Zaroff has read Rainsford's famous book "about hunting snow leopards in Tibet", and so is familiar with the author's expertise in the field.  Zaroff recognizes that, with his knowledge and experience, Rainsford would be a formidable opponent in his sinister game.  He looks forward with intense anticipation to test his wits against such a worthy foe.

Zaroff is such an accomplished hunter that he has become bored with the sport.  He has hunted every animal known to man, and has won every time.  In his search for an adversary which would provide him with more of a challenge in the hunt, Zaroff has come up with the macabre idea to hunt the only creature who, because it possesses the benefit of reasoning ability, has the potential to make the hunt more interesting to him - man himself.  Zaroff, who has been capturing anonymous sailors and using them as prey, is becoming bored with even them, however, as he has found that "they have dull brains to begin with, and...do excessively stupid and obvious things" as they try to escape the hunter.  Since Rainsford is world reknowned for his expertise in the sport, Zaroff knows he will provide a definite challenge for him as the hunted, and is looking forward to a stimulating and interesting chase.