Some diseases/medical & mental conditions affect effect body chemistry and therefore can affect mood. And the symptoms of many diseases/medical conditions make people anxious, depressed, euphoric and or/irritable. Take for instance heart disease. Obviously this can depress and even anger many people. Bipolar disorder is another example of how brain chemistry can affect mood and cause wide mood swings. Brain tumors, dementia, mental illness other than depression all affect mood because of the pathophysiology of each. Having a terminal disease is probably the biggest mood changer of them all. People must go through the 5 stages of grief, which in this case would also be anticipatory grief.
Cultural and religious beliefs can play a large role in how one responds to illness. Having or not having a support network is also a factor. Even one's individual attitude about illness affects one's mood.
In recap, some of the mood swings are chemical in origin and are internal and others are the result of one's reaction to an illness and how one copes (or not) and are external.
