I have been called “grumpy” at times.
From my perspective, as the person inside of the grump bubble, I have only this to say:
If a tree falls in a forest, and no one is around to hear it, does it make a sound?
Likewise, if no one is around to witness my grumptitude, am I actually grumpy?
Answer: NO. Grumptitude is an observed phenomenon.
Oftentimes my grumptitude is simply a product of my not wanting to interact with others. Not because I hate people, not because I’m mean, reclusive, anti-social, or anything like that…in fact, I am generally quite cheerful, social, and people-oriented.
Unless I’m not.
There are times when one needs a bit of down-time… some “me” time. Having introspective time to oneself is a way of finding balance. It’s a way of allowing whatever moods and thoughts and feelings inside to stir naturally, without distractions. It is a balancing and realignment period. A period of self-reaffirming.
For me, there are times when the germs of a creative idea are asking to be given a chance to emerge from the subconscious. I sense this need, even through I may not realize what is causing it. All I know is that at these times I have no desire to generate language for anyone who exists outside of my brain. And so I close down, become unresponsive, and let my internal world germinate whatever it is that needs to germinate.
Now, have I convinced you that I am not a grump?
I don’t think it’s possible. Because from any perspective other than my own, I’m the Grumpmeister. Solution? Simple. Go away. You and I both will notice that my grumptitude will disappear as soon as you’re gone.
Chances are, when you come back, my batteries will be recharged and I’ll be ready to party.
Earth...Brains...Technology...Design 
I know exactly what you mean. Explaining it again and again was becoming very tiring and time/thought consuming. Now I have a standard answer of “I’m fine, trust me – I would say if I wasn’t, be patient, observe and judge me on the evidence you gather”.