I didn't justify it. That's what "disposable income" is about: The money you have left over once all the bills have been paid, is yours to spend without justification.
Justifying the cost of a PPL suggests that getting a PPL is somehow a rational, economical decision. "With a PPL, you can do X or Y cheaper, faster or more efficient." It's not. Learning to fly is about passion, just like most other hobbies.
So don't justify it. Enjoy it! You only live once.