Oftentimes people can't draw the line between being nice and overly pleasing. Many people are taken for granted because they are the nice ones who wouldn't say no to anyone's request even if it's at their own expense.
These 'people pleasers' have a mindset that it's never nice to say "no" because they want to be liked and think saying "yes" is the best way to be liked. Well, sometimes saying "no" could be the best thing for you and your peace of mind. But you don't have to go about saying "no" harshly rather use ways that are subtle and comfortable.
Writing for Beliefnet, Daylle Schwartz tips on how to get comfortable with saying “No!”