NO means to decline or refuse to something. Two tiny letters but so deep impacts that some people are ready to walk on fire rather than utter this word from their mouth. 

Or we could say its so difficult for them that their nervous system doesn't allows or instructs the mouth to open.

Its so frustrating when you really want to say NO so badly that the entire universe helps you with all the possible energy but when you open your mouth its a YES !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

For example: 

You always end up in  trouble or you yourself do not have time but because of the fear of saying NO you end up burdened with all the work that you need to do whether its for office or family, neighbor or friends etc. etc.

Try to Practice saying NO at your home when you are alone. Rightly said charity begins at home. Some people are socially awkward they can say anything at home but not in public.

Reasons could be :

Trying to please everyone 

Fear of Judgement

Peculiar personality trait

Overthinking that the person might get hurt

Fear of Rejection

these are some major reasons that we face when we deal with such behavior. All these things will make you resentful and upset if you do not stop doing it.

Saying NO doesn't mean that you are a bad Person. These are just unhelpful beliefs that make it difficult to say NO.

As a child we generally develop opinions based on what our elders tell us to do and saying YES is one of the way being polite, But its very important to understand what is right and wrong.

As adults we become mature enough to decide for ourselves. Sadly, sometimes we continue our childhood beliefs and are deeply buried with fear of being humiliated or left out alone.

You have to come out of your comfort zone and find ways of starting to do what your heart truly feels.

Don't lie and don't apologize for being yourself. You can put it politely as “ Thanks for asking ” rather than directly saying no.

Prioritize the things in life where you need to say what. Don't be stressed out on these things and free your mind. You can also have excuses but that doesn't always help. 

Lead a healthy life with peace at mind and happiness at heart