If you say you're a vegan, you're committing to not eating meat or dairy. 

All of the time. 

No exceptions. 

The moment you eat meat or dairy, you cease to be a vegan. 

'Vegan' is a label.

Such labels turn decision making very easy.

"Hey, do you want cheese in your sandwich?" "No, thanks, I'm vegan!"

You don't have to think twice as there are no exceptions. 

When we commit to behaving in a certain way, firstly, it helps to make the commitment to behave that way 100% of the time. It is a lot harder to be vegan 98% of the time as you have to evaluate each time you make a choice whether it falls in the 2% or the 98%. And you have to keep track that your decisions are actually following the 98%:2% distribution. 

That's a lot of cognitive overload, and before you know it, you're no longer making a vegan choice even half the time. 

You're in the downward spiral.

What's even more effective than making a commitment to behave in a certain way a 100% of the time is to turn that behaviour into a part of our identity. Something that we are proud of. 

Rather than simply commit to not eating meat or dairy a 100% of the time, by saying "I'm vegan", you're turning that behaviour into a part of your identity.

And we find it even harder to transgress our identity. As that means changing our identity. 

Labels can be very effective when used appropriately.