English: The Blue Beryl – tree of diagnosis plate (Photo credit: Wikipedia)
Labels can be useful. Or they can be destructive. ‘Lay’ labelling someone, diagnosing them, with certain conditions such as Aspergers, ADHD, ‘a bit bipolar’, sociopathic, etc, has become increasingly common.
We DIY diagnose because it makes people’s behaviours less scary to us and makes us believe solutions are round the corner. I made a grave error in this way once. I unofficially diagnosed someone. Now, I am not saying I was entirely mistaken but I was convinced that once we knew ‘the facts’ we could find ways to deal with the situation. What actually happened is that it drove a wedge between me and another person who saw my insistence that this was the key as insensitive and simplistic.
But some times a little DIY diagnosis can be useful when other people’s behaviour mystifies us.
Borderline Personality Disorder was mentioned to me in relation to a person I know. If a friend is BPD then I should have more compassion. What if friend’s simply not a nice person – do I ignore them? If I tackle friend about behaviour will they verbally lash out at me? Do I turn the other cheek? Wait it out? Write to them? Or is it a case of Love Must Be Tough? How many times do you reach out to someone, and be rejected/pushed away, before giving up?
Have also been reading on the Myers Briggs Personality Test. Now that does seem to provide a window on the world of personality traits/types.
Just A Closer Walk With Thee 
I agree, Michael. How we see ourseoves and how others see us are often very different!
Myers-Briggs can give some real insight into oneself. Requirement, however, is being honest about who you really are and not putting who you want to be. Some people also project different personalities when working and when relaxing.