Sadness can arise from various causes as far as I understand. Sadness can arise from not getting what we want, frustrated desire, grief, loosing something we considered to be ours, such as a loved one, etc. Sometimes sadness can arise from boredom, seeing the futility of life. Sometimes it can arise when our compassion is unbalanced and falls into grief not supported with equanimity.
Depression. Sometimes that can come from feeling overwhelmed, not believing in our capacity to deal with challenges and Dukkha of life.
These are states of mind. Depression is a state of mind that has been created by feeding the hindrances. Sadness is a state of mind that also comes from feeding the hindrances. Sometimes, as I said it's because we don't get what we want or what we see, is opposite to what we would like the world to be like.
Why do you think they are treated as unavoidable diseases? Because perhaps most people don't know of any ways to cure sadness and depression, so they are seen as unavoidable, because Dukkha does exist in this world. We can't change aging, disease and death, or the loss of loved ones. Perhaps that's why they think they are unavoidable because all human beings will meet with these things in their life and if they haven't got the power of the mind to deal with it, they will normally fall into depression. Sometimes when there's a great deal of loss in a person's life, the result is sadness and depression. Oftentimes it comes from not believing in our capacity to deal with it all.
Once established as a norm in our mind, do you think we can cure them with Vipassana practice? Yes. If I didn't, then I would believe that the cycle of conditioning was permanent and there would be no way out of suffering. But since we have the good fortune to have heard the Buddha's teaching and heard the Four Noble Truths, we know there is a way out of suffering, whatever suffering comes. Especially if it's a mental difficulty arising from our reaction to life. I do believe that there is the capacity in human beings to end that suffering if they are given the methods and techniques that help them believe in themselves. The methods to learn, how to see clearly the life as it is and tools to develop wisdom. Wisdom is the cause for the purification of the mind.