[jjbinx007]

Imagine there are 100 doors. You pick one at random. You have a 1 in 100 chance of getting the right answer.

If the host opens all the doors that contain the booby prizes bar one (in this case 98 goats) and leaves one door remaining, plus your original guess, it's pretty obvious you are unlikely to have guessed correctly on your first try.

Scale it down to 3 doors and the host always revealing a goat and you have the same situation except there's a 1 in 3 chance you'll win by sticking with your first guess and a 2 in 3 chance if you swap.