This one has been in the pipes for a while. Actually, since May 2014 when I first tried a 30 day paleo challenge and my identity changed from someone not caring about food, to someone caring and healthy, putting long term health and clearer mind over (false) short term satisfaction. Today I got reminded about it again by Helen Tran here.
Basically, how you see yourself comes with certain behaviors and believes that you don't really need to influence/actively need to decide. Or as Helen phrased it "identity already defines a certain set of habits."
So, let's assume you consider yourself a healthy person? Guess you aren't smoking a package a day.
Consider yourself an athlete? I would assume you do sports a couple of times a week.
The great thing about it is, it also works the other way. Just like when you are happy, you are likely to smile. However, when you smile, it is pretty hard to have negative thoughts. The loop works both ways.
So, you can actually change your identity/how you see yourself. Taking smoking as an example. Over years, every time there was the decision "should I have a cigarette" you most often opted for "yes", making another mark on the "yes" side of the balance sheet. Once you start adding some "no's" to the balance sheet and actually outnumbering the "yesses" (obviously in a more recent time frame not in the history of time scale), you could make the argument "I am a non-smoker". And what do non-smokers do? Well, for starters, they don't smoke.
Let me even take it a step further. You are not making most of your decisions. I assume you understand that humans think they are rational, because we have consciousness, but obviously we are not rational and sth like 80% of the decision is already done once it hits your consciousness.
What I mean with that, imagine you have a judge in your head who is basically on auto pilot and only looking at the most recent balance sheet. If you help the judge (yourself) and show her that you are a non-smoker, the judge will vote for the non-smoker and make the decision of "no" for you.
Technically, by showing good faith, you are helping yourself, because you are helping the judge who makes the decision for you.
Over time, it is not only the judge temporarily, but your new identity or perceived identity (I mean, what is 'identity' after all) and you will behave a certain way by default, i.e. it's not hard to say "no" to going out, because the new "you" is not interested in it or more interested in things that don't align with that.
Copying from Helen to finish it:
"Changing your identity changes your mindset changing your habits which changes your results."