My exdp and I fell apart after 6 years due to his drug addition. Me leaving put him on an even greater path to destruction and now, thankfully, he's half way through a year long stint in rehab. He's been clean for over 7 months and he's a totally different person.
But I do believe that addiction is always there, it's in all of us, some more than others and for different things from food addictions and rage responses to narcotics and alcohol. Substance abuse is just a coping mechanism and the best we can hope for for those we know with unhealthy addictions is that they find the help they need to reprogram their coping defense mechanisms to something more grounded and productive.