I also love celebrating. Yes, the achievement is its own reward, yadda yadda. But, dammit, I still want a nice dinner and a pie on my birthday, I still want a gift under the Christmas tree, and I still want my family to recognize and celebrate my accomplishments. It reminds me how much I matter to them. And it's fun! It's even productive fun, because it is done in recognition of hard work and doing good things, which then provides an incentive to do more hard work and good things! I love making people feel special -- especially when it's for doing something good -- and I love it when they return the favor.
IME, some people are happiest with a daily routine, and others prefer ups and downs. I am in the latter category. So I will happily take any excuse to celebrate, because it breaks the routine in a good way and gives me the chance to create a happy experience for my family.
But don't fall into the trap that "celebrate" means "buy something I want." That just establishes in your own mind that frugality is something you are suffering through to get to FIRE -- it's like a diet, a temporary restriction that you are just white-knuckling through to get to a goal, at which point hallelujah, you won't have to worry about it any more. And that approach is just about as effective long-term as any other diet.
The point is two-fold. First, you need to make sure that your split between spending and saving is something that you can sustain long-term. If you constantly have a running list of things you want to spend money on, maybe you're cutting it too tight and should work a little more "stuff" into your normal budget. And second, when it is time to celebrate, think of non-spendy things that you would enjoy. One of the things I do is make a special dinner for DH, with his favorite things -- or if we were going to go out to dinner (which is part of our budget, since it's something we enjoy), maybe I'll make a reservation at a sushi place, because even though I don't eat fish, he adores sushi. And honestly, one of the best ways that he can treat me is to manage logistical stuff for me -- I really don't care where we eat, if he makes the reservation and gets the babysitter, I am already on cloud 9. Last year for Christmas, I said I'd really, really like it if he could take my car to get the oil changed and the tires inflated and the car washed, and do one or two other similar things that I always find a reason to put off. One of my best Mother's Day gifts was when he took the kids out for hours and let me curl up in my chair with a book and the clicker. Etc. etc. etc.