1 is divisible by itself, which IS 1. There's only one divisor, unlike prime numbers, which have two.
*Shakes Fist*
<Homer's Voice>Stupid 1! Why do you think you're better than everyone else!? "Ooo, look, I'm number 1, and I only have ONE divisor, they're sooo out of style!" Damn non-prime number.</Homer's Voice>