From the book, "Khasais Al-T'abir Al-Quran wa Samatuhu Al-Balagha" by Dr. Abdul Matin Al-Mut'i Al-Azhari, and explained to me by Imam Suhaib while jogging (so if I get anything wrong, forgive me, but I was trying to be healthy):

The word ya, in Arabic, is used to call to someone.  For example, if you were in a room and your friend, lets say his name is 'Azam' was in the next room: how would you call him?  You would say, "Ya Azam!"  However, if Azam was on the other side of a football field and you wanted to call him, how would you do it?  You would say, "YA AZAM!"  However, if our beloved friend Azam was sitting right next to you, extremely close to you, how would you call him?  Of course, you would just turn to him and say, "Azam."  THIS is the closeness that we share to Allah SWT!  The scholars of Balagah say the ya used for calling of Allah SWT to His servants is hidden in its languge to illustrate the intense and extreme closeness Allah SWT shares with them.  In fact, if you were to look at every single supplication made to Allah SWT in the Qur'an you would only find the word ya used twice!