I covered it here:
http://vitalcoaching.com/stayfree.htm
http://en.wikipedia.org/wiki/Unrequited_love
Is unrequited love an addiction?
Yes!
You are emotionally addicted to someone and feel you can't live without that person!
Yes!
More links
Personal experiences with unrequited love
Unrequited Love Support Group
How unrequited love can be an illness, or even fatal (BBC)