It’s easy to justify an indulgence if you say to yourself, “Just this once”.
We make this justification when we want to enjoy something that has a cost which we don’t want to pay. That’s why we say, “Because it’s Friday night”, or “It’s been a big day”, or “I’m on holidays”.
This justification is valid if an indulgence really is infrequent.
But unfortunately, you are the easiest person to lie to.
If you are doing something “once-off” more than once, it’s a habit. And if it’s a habit, it means you are paying the cost, despite the justification.