I would tell them that nothing could be further from the truth and assure them there is room for everyone.
I would then start qualifying them.
"Are you an alcoholic?"
"What does that mean to you?"
I would begin sharing my story, concentrating on the allergy/obsession.
"Does any of that sound familiar?"
"Give me some examples"
"Have you tried quiting before? Tell me what happened"
In short, I would try to help them identify in, instead of compare out.