Why do we tend to become grumpy when we get old/become elderly?

Question

Why do we tend to become grumpy when we get old/become elderly?

Answer 1

They realize that they have made no impact upon the world.  And that for all of their patience and turning the other cheek- hasn't made one whit of difference.

Answer 2

I agree with the first answerer. Also, your body starts to hurt and breakdown, you are more prone to various illnesses like Diabetes, you're bored, and may be a little lonely because your friends are either too busy with their own families or dying.