Have often been asked this over the decades of my life. This question has multiple answers for me.
1. Nothing to say at a given moment.
2. Don't feel like talking.--Not being rude, just not up to it at the time.
3. Too lost in my own thoughts at the moment to handle conversations.
4. Not into social stuff, as I am an introverted hermit, and talking involves socializing.
5. Too exhausted from my chronic fatigue to handle conversations.
I tend to try to give the least offensive answer to this question, as I found out decades ago, that NTs tend to take offense easily, if you don't socialize heavily with them. The fact that they are imposing their socializing on me, someone who just wants to be left alone, and demanding that I socialize back, offends me, but oh boy, never tell them that.
I've noticed in my decades of life that NTs often get offended by the strangest things, and it's not always possible to avoid offending them because of this. I'd rather stick to being a hermit. NTs are too hard to deal with on a full time basis.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau