Many adults have multiple casual friendships but no genuine 'best friend' — someone you can call at 3am, share embarrassing moments with, know your weaknesses, and still want to spend time together. Research consistently shows the deep mutual relationship has measurable health benefits casual friendships don't replicate.
What 'best friend' actually means
Mutual deep knowledge over years. Honest enough to challenge you. Available for big moments. Reciprocal investment over time. Not 'most fun to hang out with' (that's casual friendship).
How to build or maintain one
Existing best friend: protect the relationship through life changes (don't let job/parenting/move erode it). No existing best friend: invest deliberately in 1-2 of your closer friendships over years. Genuine vulnerability is the path.
Best friends in your 40s and 50s tend to be those you committed to in your 20s and 30s. The investment is long-term; the benefit is lifelong.