Evolutionarily, it’s important that humans have said they will not, under any condition, eat a poisonous food or visit a nearby village known to be violent. In present day, however, humans constrain themselves and restrict their lives verbally through words such as ‘never’ and ‘won’t’, which at some level contributes to a lack of happiness.
What you really want are guardrails. These are hard to establish and modern humans often resort to extremism as a means to rid of certain behaviors. Extreme limits can induce pride and build ego, which only bolster what really should be guardrails in the first place. Ultimately, any means of stopping behaviors that doesn’t accommodate for still doing the behavior itself is a slippery slope that few can resist towards embracing the behavior.
It is easy to say you never will do a behavior, but harder to say you might. The ruminating possibility that you could indulge in a given behavior creates the illusion of negating all your efforts thus far, but this is not the case. Maintain guardrails in your life to the extent that they benefit you and provide a level of structure, but stray from extremism that portrays what you don’t do as who you are.