Let's say your manager has assigned a project to you. You're already working on a few priorities, but you accept this anyway. Why? Any one of a number of reasons. Maybe you think the project is going to add to your skillsets, or you want your manager to know that you are willing to take on new challenges, or you just can't say no to your manager. Whatever the case, once you've started the project, you realize, you really don't have the time and resources to deliver. So what now?
Why is saying no so hard? Maybe you have the incessant need to please people a la Monica Geller in Friends, or maybe you're just too scared. Either way, by saying yes to everything, you might be stretching yourself too thin and taking on more than you can actually handle. Even if you aren't dropping the ball yet, continuing with the "never say no" rule could hurt your career.