"73 de W1AW" = "talk to you again soon, from radio station call sign W1AW"
He likes it because 73 is the 21st prime number with it's mirror 37, which is the 12th and it's mirror which is 21 and is the answer to 7x3.
He likes it most because 73 is a palindrome (1001001).
Leonard: "73 is the Chuck Norris of numbers"
Sheldon: "No it's not, all you get from Chuck Norris backwards is Sirron Kcuhc"
Aaron: I have some from Mike too... at least it was fun
Caren: That's for true :] way better than 69ing
Aaron: Or even the lazy dog!