Making mistakes and teasing each other are like your common necessity in keeping the friendship going. Friends give you joy and happiness and put a big smile on your face when you're down and gloomy. They'll in fact sing you a song to cheer you up, giving you hope that no one else can.
Sometimes, life is like a rocking boat, imbalanced and uncertain. Most often we see life in its brightest side and sometimes we don't but friends are the ones who can shed the light on your ambiguity and confusion about it. Regardless whether they give you appropriate advice or not, its the effort that counts. They'll show you how they can help you in any possible way they can despite the finite strength and wit they have within them.
If you feel a big burden on your shoulders, they'll lend you their shoulders instead and alleviate the burden to become lesser or even none. These are merely my thoughts and description on what friends are like. Maybe different people have different ideas of what friends are like. But to me, friends can help you, amend you, train you, give joy to you, and lighten up your day. Friends are forever.