Let's be honest with each other. Most of the time people interested in FIRE are somewhat like outliers. The idea itself is quite unpopular in the general popular and may sound extremely weird at first. It is also challenging to pull off, requires a lot of effort and goes against the grain of the modern society, making it harder to fit in.
As a result, FIRE people don't talk about FIRE with ordinary people much and, thus, usually have very few contacts interested in Financial Independence. This definitely makes sense. Finances are somewhat of a taboo topic (especially for people who have troubles with them) and talking about saving a bunch of money may sound like a boasting. Also sharing that you have or plan to have a sizable chunk of money does not sound good. As a result, even if there is a mustachian in your community, it is very unlikely that you will discover each other (this happened to me, I learned that my friend was interested in FIRE after knowing him for a year, see details in my previous post on signaling about FIRE).
At the same time, going through this journey alone would be too sad. I will eventually share my ways how to find friends interested in Financial Independence and Early Retirement in my other post (TODO), here let's see my reasons why friends who are into Financial Independence are great:
You get new ideas. We are diverse, each person has their own experience, situations and problems, but we also share some of them. With mustachians, you can bounce your ideas and learn something new. This often involves how to save more, invest better or some interesting information sources. You can also see what makes them happy and what they optimize for and use that as a point in your own views.
You can openly discuss finances. FIRE people know their asset allocation and many of them even have their investor policies. They are also not shy from numbers and discussing concrete stuff. While revealing absolute values may be avoided (but some people even go that far), you can still discuss a lot with percentages.
They understand you saving money. We are simple and understanding bunch. If someone does not want to go to a restaurant, because they find it too expensive, this is perfectly fine. There is nothing wrong with that. We all have our own "too expensive areas" and thresholds. Unfortunately, ordinary people often just find you weird and most of the time you can't even openly tell them the reason.
They are interesting. I personally noticed that each FIRE person I met have their own story and charm. Perhaps I just don't look with that level of details at general population, but somehow I just click with FIRE people much better. They have goals, usually unconventional views and I love to get to know them.
They are likely frugal. As a result, there are many more activities you can do together and be overall on the same page about money.
You share more in common than you think. I noticed that some special kind of people are being drawn into FIRE. I haven't yet identified all the features, but I would call them implicitly rational and scientific, unconventional, being ready to go against status quo, having and trusting their own opinion, often having peculiar hobbies. As a result, FIRE unites people. It is much easier to find common language and build friendships with people interested in Financial Independence and Mustachianism than a random person from a street.
Once you FIRE, you can hang out together while non-FIRE people work. Very like people from your old social circle will be busy most of the with their own work. Thus, there is a risk to be lonely, while your friends work. You don't have this problem with FIRE friends, because you both are financially independent :)
These are just my own observations, which I made after talking to many dozens of FIRE people (including on this site, but also various events). I am pretty sure that there are many more arguments to have FIRE and mustachian friends.
TODO: add a link to a post about how to actually find FIRE friends, e.g. using this site.