We all have many biases towards ourselves. They helps reduce stress and keep our heads held high. These include the self serving bias, attribution theory, and psychological projection. Since we are constantly judging each other more harshly than we judge ourselves, it would make sense our friends, despite looking past our faults enough to hang around us, would be the best at analyzing our personalities.
The study on personality and longevity from Washington University in St. Louis. was published earlier this year in The Journal of Psychological Science.
"The study demonstrates that your personality at an early age (20s) can predict how long you will live across 75 years and that close friends are usually better than you at recognizing these traits." Science Daily
Male participants seen by their friends as more open and conscientious ended up living longer. Female participants whose friends rated them as high on emotional stability and agreeableness also enjoyed longer lifespans."
These predictions rang true regardless of the current state of health exhibited by the individuals friends. The friends were able to identify characteristics that could prove dangerous down the road such as inclinations towards depression and anger.
Men who are conscientiousness are more prone to eat right and avoid risks, while women's emotional stability could help them fight depression and anxiety.
The study is longitudinal and followed young people from the 1930's, most of whom were getting married. It measured both self-reported data and that of their friends (bridesmaids and groomsmen). The personality results have since been compared with death certificates to see how they correlate to length of life.
This is one of the longest psychological studies ever performed and the results today would likely be different judging that females would likely share more characteristics with that of men's personalities today, given the changing gender roles since then.
So don't be afraid to ask your friends to be honest with you about areas of your personality you could improve, it could save your life (10 or 20 years).