MATH QUESTION ABOUT THE MOVIE 21

ok umm have you seen the movie 21! ok well even if u havent u can help. when ben is in cl. his professor micky asks him abou the brobability of a care behind three doors. 2 have goats. he picks one. professor open three and there is a goat. he asks ben do u want to change to door 2. he says yes. WHY>?????????? i dnt get it probability means it can be any??????