When you first meet someone, your best self is at the forefront. You're very proper, and you inquire politely about the peripheral aspects of the other's life. It's a delicate dance that both of you adhere to, and you're both careful not to scare the other off just yet. You're careful not to say something that would cross the line far too early.
Then you get increasingly comfortable with the other person. You're both still cautious, but what you talk about delves deeper into the sources of your personalities. Then, there's a tense moment when you let yourself finally share the quirkier aspects of yourself. You test the waters and see whether they minded the terrible pun, or off color joke, or bold claim.
And when they don't, you internally breathe a deep, cathartic sigh of relief. They respond in kindness, and you enter the second stage of your relationship. There are very few presumptions and very few formalities. Both of you immediately begin revealing your true selves to one another.
They drop by your room with only the text "I'm coming over" and feel comfortable raiding your pantry for food. They feel comfortable confiding in you their deepest fears and worries about the future, and you judgmentally listen, and you do the same. You both are comfortable in calling the other out when they're making a bad life choice or are fooling themselves. You give tough love and get tough love in return.
Once you're good friends, you're able to relax around the other person. Once the fear of rejection is overcome, there's an undeniable air of comfort that settles between the two of you. You can both see each other at your worst and love each other just as much as when you see each other at your best.
Once you're able to be yourself around a new friend, you can finally begin growing together and you've finally forged a true friendship.