Resisting an addiction can be nearly impossible for some people, but the damage it can do to a relationship can be impossible to repair. Some addicts eventually realise this, and they know they have wandered down to the last thread of their relationship. They can see one more episode will sever the final thread, so becoming an adult and resisting their addiction is their only option. Their partner might have given them an ultimatum, but it is just as likely they have foregone that formality and are simply planning to leave.
Addiction when a person is dating can be easy enough to hide at first, but it can eventually surface. Those without a legal commitment or marriage will often walk away from a partner unable to exert control, and it could be a continuing situation for the addict. They may resent the other person leaving, but it may not be enough for them to do anything to curb their issue. Rather than going through the painstaking process of rehabilitation, they could simply decide the other person was not right for them.
Long term relationships today often result in marriage, and this is where an addiction can be a major issue. A person able to hide their problem may end up married, and their partner is suddenly surprised to find out drugs, alcohol, or gambling is more important to their spouse. The shock alone could push them toward divorce court, and they might see it as a breach of the relationship because their partner never told them what was really going on. A lack of trust can quickly develop, and there might be no way to repair the damage.
When a person stays with an addicted partner, it is often because that person professes a willingness to undergo treatment. They may understand it could take time to overcome the issue, but those who continue to relapse may just lose their last chance for a happy relationship by giving in to their addiction instead of finding the discipline they need to overcome it.