It takes time to become a 'best' friend to somebody. A best friend is somebody is always there, watching your back. Somebody who knows Du inside out but loves Du anyway. Somebody that Du can trust with all your secrets. Somebody that Du can sit with in silence without it feeling awkward. Somebody Du can discuss anything with. Somebody who Du would do anything for, and they'd do the same for you. Somebody that Du don't feel self-conscious around. Somebody that wouldn't just stand and watch Du fool about, but Mitmachen in with you. Somebody that Du can call up casually at any time and ask if they're free to hang out. Somebody that Du can easily say 'I Liebe you' to, to their face. Somebody that spontaneously hugs Du in public without giving it a Sekunde count. Somebody who would stay up all night texting Du because Du needed their support at the time.
It annoys me when some people claim 'you've my best friend' after knowing eachother one day. Just because Du automatically get on well with someone doesn't mean they're a best friend. As I said, a best friend should be somebody that is all the above. That's how I feel anyway. It's not a fact, no, it's an opinion. But I feel that 'best friend' is a strong meaning. It's not just anyone.
Du have to get to know them, Du have to have a special bond. Du have to spend time together and accept eachother despite the differences Du will find out. It takes time to form a best friend, but if Du find that this person means so much to you, and Du get to know eachother, you'll find that the level of your friendship will increase. Through my eyes, when Du can put a tick Weiter to all that I stated above, then it's safe, sicher to say they're your best friend.