You've taken your mom to the doctor and she's upset with you because the doctor's office was cold. You've helped your dad with the yard and he's annoyed that you didn't mow the grass in the right pattern. Why do many elders complain about everything you do?
Much depends on the parents' personalities throughout life. If your parents were the bickering type and were always negative, this complaining may be the only way they know how to communicate. They may not even be aware how their attitude affects others. Since you grew up in their household you can ask yourself, "Is this how they always acted?"
I'm not suggesting that their continued negative take on life, especially the things you go out of your way to do for them, is okay. However, realistically, you aren't likely to change their personalities. The physical and mental frustrations that go with aging are likely to intensify an already negative personality, so you already know what you are in for.
For some, however, this negative mindset or constant complaining is new behavior. Your mom was always sweet, almost timid. Now she's a bearcat. Your dad was always jolly and supportive. Now he's controlling and angry. When these changes occur, the adult child caregiver has a better chance to uncover the reason for the changed behavior and perhaps do something about it, so we'll look at those reasons first.
Medications Can Cause Personality Changes
Just as an antidepressant can balance a person's body chemistry to help the depressed person emerge from a depressed state, an antidepressant can, if it's the wrong one for that person, make the person's illness worse. Thus, if your loved one has started an antidepressant, don't just assume things will get better. The drug may help quite quickly or it may take several weeks to have any effect. And that effect may be positive or negative. It's possible that this drug is the wrong one for the individual taking it, and will need to be changed.
Other medications can also have negative effects on personality. Anti-seizure medications, blood pressure medications, even anti-inflammatories can cause personality changes in some people. Drug interactions can also cause problems, so if your loved one has several medications, the combination should be checked by a pharmacist for interaction problems. Many an elder has been thought to have dementia, but once off all drugs, the dementia suddenly disappears. Drugs should always be suspect in sudden personality changes.