Is it common for alcoholics/addicts to leave their SO for someone else?

I was having a discussion with a friend & he said, "They drink & do drugs because they're not happy about their life & the people in it." I said "ok, but what if the family/spouse is very supportive?" , He said " it doesn't matter, if they were happy, they wouldn't drink or do drugs".....my friend had a father who was an addict & deserted the family for a woman he met in a support group. He said "this is common"....