...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.