How is it that my friends become best friends with certain girls more quickly and easily than I do?

Because whether you realize it or not you don't want to be her friend as much. What the mind wants it will achieve. Inside maybe you don't feel a close connection to that girl. She may feel it with you but unless it is a mutual and genuine feeling you wont become closer friends. It could mean that your standards are higher or you are more picky, but one day you'll find somebody you really can relate to on all levels and understand everything i am saying more. I think you may think your not going as far as them but will end up going the farthest. Goodluck.