Because we hate the addiction, but we still love the kid. I am having similar issues with my 19 year old daughter. She isn't an addict, but she tries to kill herself every other month. I told her I can't deal with this anymore and she has to find another place to go when she is released from the hospital this time. It's tearing me apart because she's really a good kid; she just makes some extremely bad choices.
My heart goes out to you. Be extra gentle with yourself but stick to your guns on this issue.