I think it's the addiction in general more than the alcohol.
I quit smoking for seven years. Hated the smell and all. Then one night I was drinking and told a friend to give me one. I could handle just one or two while drinking right? Ended up smoking his whole pack and bought another as soon as I got up the next day. Just like I had never quit I was right back at it. Still smoking now 11 years later. I hope I never forget that lesson.