My definition of health is how well your body is functioning, and if it has anything adversely effecting it that makes it deviate from the norm, which would indicate a lack of health. My idea of illness is the what prevents you from fully functioning or what causes the problems. So the deciding factors on whether to include sadness as an illness was basically if occurred for a specific reason, or if it was a completely random and general feeling. The way I see it is that if you are sad for a specific reason such as a death in the family, then it is not a illness exactly, but a natural process that you must go through in order to cope. This would make it a healing mechanism. The other option is that it is a general feeling of depression, which is in my mind a different issue. This could be a symptom of a different problem, such as a chemical imbalance, or it could be your subconscious alerting you to the fact that you are not satisfied with your life. In this case I would say that it is in fact an illness.
These ideals mostly came from my family, and after that from society/culture. I believe that the largest influence in a persons life and ideals is from their family with the culture being the second most influential. Personal experience also plays a role in this, but most of the personal experience was in the context of family so it is drastically effected by how my family reacts.
I consider anxiety to be in a similar position as sadness in the context that it can be an illness if certain criteria are met, but otherwise it is just a natural response to external stimuli. I had a bit of trouble classifying menstruation, since as a male I have a bit more limited outlook on this, but in general I believe that it is not an illness, but is instead a natural biological function. Even being a biological function and serving a purpose, it still would need to be evaluated on a case by case basis to see how it effected the individual rather than just saying whether it is a illness or not. In my mind old age is most definitely not an illness. Although it has an adverse effect on your life, it is a natural part of life that all organisms must undergo and being old isn’t necessarily bad.