We, human being, are social animal. We cannot live an isolated life in wood and survive on our own for a long period of time. Whether we like or not, but we have to live in a society for our survival and happiness. In society, our happiness is interdependent and we all have to make room for others to be happy.
However, the primary goal of a person is their own personal development and for that they need to know how to handle complexities of relationship while living within a society. Sometimes, saying a simple "no" politely can save a person from future regrets and unpleasant situations.
The habit of saying 'yes' at all times is an open invitation of trouble. Saying 'yes' to each and every person, advice, proposal, request and offer that come on the way can increase your work pressure and keep yourself occupied at such a level that your own goal does not get the desired attention.
The biggest drawback of being ready to help others all the time is that people keep on putting expectations on you either it is your personal or professional life. And you will always remain under the extra burden of expectations leaving behind the dream of realizing your own goal of life.
Often, most of the people form a habit of saying 'yes' all the times because of their desire to satisfy the need of others to get praise from them in return. However, the fact is that people expectations are unlimited and no one can fulfill each and all expectations of a single person. So the desire to get praise from others is temporary in nature, and it comes at cost of sacrificing own expectation from life.
Occasionally helping others in times of their need is considered a kind gesture; a positive sign for the survival of humanity. However, every person has hidden ability to get over from almost every problem by themselves, and it is their responsibility to do so. So while helping others you should make sure that other people do not have expectations from you to get bail out from their problems every time.
No doubt saying 'no' is considered rude and the chances are some people may feel hurt but the positive aspect of it is that it can save a person from many complications. Often the ability to say "no" saves a person from embarrassment, disappointment, regrets and increases their level of happiness.
Hence, when saying 'no' is in a best interest of all, be tough to say so politely without any kind of guilt feeling for hurting others. It is the love for self and dedication towards the realization of ultimate goal of life that enable you to say "no" politely for your own benefits and society at large. So if you do not want to create obstacles in your happiness and have peace of mind, you should learn to say "no" politely.