It sounds bizarre to choose to spend your free time alone when you have a plethora of friends who would spend time with you at the drop of a hat. There are some pretty important things that people don't understand about being their own best friend and some very common misconceptions.
You have known yourself the longest. You know absolutely everything that has happened to you ever. Most people that come into your life have to get to a certain point of friendship with you before you even remotely open up about the things that make you who you are. Lets be honest, we all have past experiences who make us who we are whether they be positive, negative, detrimental or not a big deal at all.
You agree with yourself on everything. Your opinions on things and outlook on life is completely original to who you are. Your brain and your thoughts are completely untainted by anyone else's opinion. After realizing this, being alone with your thoughts is completely different from before. You begin to think more clearly and rationally when you allow yourself to get lost in your thoughts.
You can do what you want to do. Aside from the mental benefits of being your own friend, once you become your own best friend you start seeking advice from within more than you seek advice from others. This gives you a sense of freedom that you miss out on when you rely on others to help you make your decisions. If you don't want to look at it as deeply as I do look at it this way: if you're eating dinner by yourself you don't have to care what anyone else wants.
You will become more productive. When you're distracted by friendships, gossip, rumors, etc. your objectives change. Your mindset all of a sudden goes from "do your homework and take a nap" to "text her this, call him and explain and make sure no one is mad." You're allowed to think of yourself first. You are allowed to be selfish. When you become your own best friend you allow for this to happen naturally.
You won't be lonely. I am by no means preaching to anyone that they should ghost all of their friends and become a loner, because that is just plain wrong. Balance your life out. Spend time with yourself when you know you need to and your friend will be there when the gaps need to be filled. At the end of the day and at the end of your life you will be the only one who was there for the entire journey.
Your self confidence will skyrocket. Self love is something that is being preached more and more, especially to young girls. When you allow yourself to have fun alone, you allow yourself to fall in love with yourself. What you do, when you do it and how you do it all make up who you are. Build a person that you love.
I became my own best friend approximately two weeks after I thought I hit rock bottom and I couldn't be happier. I have been here for myself more than anyone and I still have the most amazing group of friends a girl could ask for. Because I have found a true friend in myself it is much easier for me to detect a real friend in someone else.