Earlier quoted context omitted.
I found it oftentimes easier to say no without saying reason. The issue for me was that when I state reason, people wanted to negotiate, attempted to "advise" me on how organize my time better and so on and so forth. Overtime (at least against many people) I found it much less draining to simply insist on politely stated no without further explanation.
Counterpoint: when people do this to me, I instinctively assume that the true reason is laziness. On top of that, I feel alienated – what, you think you're so important that you can brush me off just like that, and I'm not even worthy being given a reason? Be careful with this one. It's very much possible to state a reason and still express that you won't change your mind on the matter. Although it is more effort, yo…
If somebody thinks you lazy, so be it. But you don't owe anyone a reason.
Reasons, as you see them, are often "excuses" to the other person. Mught be even worse. Now you're not just lazy, you also make excuses.
Personally I'm bad at this too. I often give reasons/excuses when there's no need and even more often I say No by not saying anything at all and waiting for the thing to pass.