Why does it hurt?

Why does it hurt when you miss someone? Is it because of the disappointments in my heart? Is it because of wanting to look at his face but cant, or is it because it's my worries for him but yet feel that I shouldnt feel this way? Well, on his 32th birthday, i only wrote him an email - "On this special day... I want to see you happy. Happy Birthday. I do love you. Wishing you many happy tomorrows." I feel amazed. I loved my email. The week after next, is my birthday. In my heart, I am anticipating something. Will there be anything for me, from him?