By saying “no”, we take a step towards uncluttering
Your thesis is we are unable to say no because we want to appear positive and we want to appear helpful. Possible. Hard to disagree with this thesis, also because that’s a view of the world that sees the glass half full, not half-empty.
I’d also say many of us are unable to say no not because we are polite or something but because we are greedy. As we assent, an internal dialogue tells us, “Perhaps that ‘yes’ can show me in a good light to the boss”, “perhaps another ‘yes’ will bring me that extra opportunity”; and sometimes we are coerced or cornered to say yes. Again, somewhere the demon of greed lurks to say that if I were to stand up for my own time, a punishment lurks in the corner. Hence …
It takes courage to say “no” in the face of our greed/passion/politeness. Saying “no” is about courage. Saying no is about realising who I am and how much I can stretch myself. As a hopeless “yes”-sayer to everything and everyone, I rest my case.