If you consider any man a friend whom you do not trust as you trust yourself, you are mightily mistaken and you do not sufficiently understand what true friendship means… When friendship is settled, you must trust; before friendship is formed, you must pass judgment. Those persons indeed put last first and confound their duties, who … judge a man after they have made him their friend, instead of making him their friend after they have judged him. Ponder for a long time whether you shall admit a given person to your friendship; but when you have decided to admit him, welcome him with all your heart and soul. Speak as boldly with him as with yourself… Regard him as loyal and you will make him loyal.

• Seneca

I have lots of instincts about what good friendship is based on lots of conversation with high school friends. I do not think people take enough care defining norms for good friendship

What comes to mind

• Non-competitiveness: They celebrate your wins
• Non-self-centered: They don't make things about themself
• Trustworthy: They do not take advantage of weaknesses you show
• Vulnerability: They share real weaknesses (something you could take advantage of for gain)

In friendship there is union based on something deeper than the material facts of your person: there is some kind of shared value system that enables trust. If you find a good friend there is no reason they should not be lifelong. Shared values transcends shared experiences, so distance (space/time) should not have much bearing once the foundation is laid out.

It is important to also hold yourself to a high standard. Treat friends how they would like to be treated. You also start to self-select people who look for the same things in friendship.