Adam Grant explains:

I learned that there’s a big difference between pleasing people and helping them. Being a giver is not about saying yes to all of the people all of the time to all of the requests. It’s about saying yes to some of the people (generous givers and “matchers” who aim for quid pro quo, but not necessarily the selfish takers) some of the time (when it won’t compromise your own goals and ambitions) to some of the requests (when you have resources or skills that are uniquely relevant). Outside those specific conditions, successful givers follow Buffett’s edict and decline for one fundamental reason:

Saying no frees you up to say yes when it matters most.