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If you assume you know what best friends' roles are, you could get disappointed and frustrated. Best friends are the most valuable friends you have, but they can't help or support you in every aspect of your life. Do not expect them to always be there, or expect them to say the thing you want to hear. If you set expectations for your best friends that are too high, it will only leave you disappointed and defeated.  Be your own caretaker. This comes back to being your own best friend and taking care of yourself. If you take care of yourself, you'll never put your friend in a position where they have to come in and rescue you. If you never put your friend in this position, you'll never be disappointed. Remember that no one is perfect - not even your best friend. Everyone has their own faults, and they need to work through it. Don't be mean to your friend about their faults, but help them work through it while asking for help on your own as well. However, when gently letting your friend know, understand that your friend has feelings, and it's best to only focus on the faults that are the biggest threat to your friendship. Know what faults to ignore/let go of and what faults to help your friend solve. Sometimes, you need to let your friend solve their faults without your help, unless they ask for your assistance. Constant nagging will make your friend feel stressed and angry, and they may be unwilling to improve your friendship if you're putting pressure on it. It's natural to stop having chemistry with a certain person. If you grow apart, for whatever reason, just be happy for the good times, and know how lucky you were to have that great person be a part of your life.  If neither of you make an effort to get together, or if you get into fights with them for no reason, then maybe you weren't meant to be best friends. It's not anyone's fault. Maybe you two are too much alike. Who knows? You might just need a break from one another for a few weeks. Always treat your friend with respect, even if you grow apart. Don't be angry with them, it's truly not worth keeping all that anger locked up inside. Be courteous, kind, and respectful if you have a falling apart. You never know when things might change. If your best friend picks another friend over you, try to be friends with their friend too. Maybe you guys can be a group of friends! Everyone is always looking for new friends. But never, ever leave the old one; your loyalty is what the other person expects and counts on every day.
Avoid expectations. Understand that best friends sometimes grow apart. Let your friend have other friends, too.