Johanna Rothman suggests that one of the shortest words in the English language, is also one of the hardest to say. So, why is it? No: Such a Difficult Word appears in her column at StickyMinds.com:
Some people can't say no and make it stick because they feel bad when they try. I've heard many reactions to my no's. One manager told me I wasn't a team player. Once, when I thought I was being a team player by explaining what I could commit to and what I could not, a manager told me I was being a slacker. No one likes hearing that your manager considers you a slacker or not a team player. [It goes on.]
Not only do managers have a hard time hearing the word "no," people have a hard time saying it in the first place. Of course, there is probably some chicken-and-egg going on here. Managers don't expect to hear it, so they react rather negatively (as above). And people don't want to be shamed by their managers or "let their people down" and so they don't want to say no either. Johanna describes the inevitable results pretty nicely.
One thing that should be clear when I decline a request is why. Do I have the capacity? Or what has to drop, if I take on this work? Does the request make sense (do I understand it)? Similarly, do I have everything I need to successfully complete it? (Sometimes this is called "full kit" for the task.)
I could probably list many other underlying reasons that are valid. But each of these should elicit a different response from the requestor - assuming they don't faint of shock. The capacity issue should elicit discussions about the current workload and whether other things can be shifted, if this new work is truly important. Or could the new work be delayed for X period until my current load reduces? If I don't have full kit for the work, when might I have full kit and what can be done to improve the chances of successfully completing the work (quickly)?
And now, for your entertainment: a boy playing and singing Tantrum from Sandra Boynton's children's book / music Dog Train. The lyrics include a lot of "No! No! No! I don't wanna! I don't wanna!" so it fits.