I understand the depression part of it from the patients standpoint, fishy. I never thought of it in terms of the caregiver---forgive me for that.
These terrible diseases change everything. Once, sexually active, it now takes an act of congress to have intercourse. With sjogrens, all the moisture producing glands are shot so it is dry as dust in those locations. Then of course finding a position you can both accommodate is tough. It sure as heck takes the romance and passion out of the act!
The pain moves around constantly every moment of everyday and makes one irritable. It's hard to see the joy in things when you feel crappy. These diseases are so tricky too---the pain travels, confusing everyone. Ultimately, despite good intentions, you become a complainer. You try not to but when it hurts, you cannot help but gasp or groan. It's a literal pain in the butt!