Most people claim they give in to sudden requests because they hate letting others down. I say it's more about not disappointing ourselves: We're hooked on feeling needed. If we take a hard look at ourselves, we might see that we unwittingly encourage people to come to us for every little thing. Interruptions can also be a welcome distraction. Faced with an unpleasant task, we're more than happy to turn our attention elsewhere. Finally, we often don't say no because of simple disorganization. In a choppy and shapeless day, we handle disruption immediately because we figure, if not now, when?
While it's important to be reasonably accessible to the people you live and work with, you don't want to spend most of your waking hours in helper mode at the expense of completing your own critical tasks. Even if you're in crisis management or, for that matter, if you're a stay-at-home mom, you need to prioritize requests. Otherwise you get trapped in a whirlwind of multitasking where you start many things and finish nothing.