I have three best friends, and they're my best friends for the exact same reasons.
We're similar. We share the same passions and so there is always something for us to talk about. Not only that, but they understand when I talk to them about it. They know exactly how I feel, and exactly what advice to give me. And even about the things that we're not so similar about - they understand, and most importantly, they don't judge. We understand that different people have different preferences and values, and that's ok!
I can tell them anything. I feel as if with most people, I have to constantly filter my words. Every single sentence coming out of my mouth has to be carefully crafted in order to make sure that I don't offend someone, or say something stupid - IT'S EXHAUSTING. With my best friends, I can say whatever the hell I want. Obviously, i'm not going to be extremely mean to them, but I can be honest. I can make stupid jokes, and they will either love them, or think i'm ridiculous (but in a loving way).
We've got each other's backs. We know that we can tell each other anything. While we won't be mean, we can also trust each other to be honest. They'll take my side, but they also won't be scared to tell me i'm wrong.
They don't expect too much of me. I like to think of myself as an "ambivert." Meaning, I like small groups and while I do occasionally like going out, I also love being alone. This is why I love my best friends - they're perfectly content with just sitting side by side, watching tv. If we want to have long, deep conversations, we can. If we just want each other's company, that's fine too.