I was diagnosed as bipolar 2 a few years ago, and I just assumed the mood swings I dealt with were normal. Doesn't everyone occasionally get too enthusiastic during a meeting? Or call in sick because they can't get out of bed? Turns out no, everyone has mood swings from time to time but mine were more predictable and severe enough they were preventing me from doing the things I wanted to. Hence the diagnosis and medication.
This is why mental health professionals are important. It can go both ways -- sometimes folks are just neurotic and overanalyzing themselves, and sometimes they're oblivious to the mental illness they do have. Medication and life changes help, but the first step is to see a professional who can help you sort through whether your mood issues rise to the level of clinical or not.
Trauma is a huge factor in mental illness as well, so much so that the definition of a “healthy individual” can only be really anchored to a specific point in time. Some things that rise to the level of clinical (anxiety, depression, BPD, PTSD, DPD) are largely seen as the result of trauma and/or some hereditary predisposition.
i.e. for parameter X which typically ranges between x and x' and for which people estimate themselves at x1, they typically severely underestimate the size of the range (x,x') and conclude that x1 is outside it, when it's really inside it.