It is inherirted, I believe. My dad was recently diagnosed with depression, but I bet if they saw him more often he could get a bipolar diagnosis. I can think of times when he appeared dysphoric many, many, many times in my life.

My mom, I know she had anxiety issues. I can think back and remember all of her nervous ticks pretty well.

I think, once you've lived with something and then learn to recognize what it is, you start seeing it in others.