I met my best friend when we were in our late 20s. We were assigned as roommates while teaching English in Thailand. I'm more introverted and she's more extroverted, so our early friendship was her finding fun things to do and me tagging along. We have different outlooks at times, but similar values, which is good because we can help balance each other out.
As for what has made our friendship so strong, it's largely because of rough times we've gone through together. We've been there for each other in good times, bad times, and really awful times. She is family. Fortunately, our daughters are also best friends now! And our husbands get along okay, too.