I don't know if this story is true, but I read one time that before he met Yoko Ono, John Lennon saw one of her works in an art gallery. It was a ladder that led to a maze on the ceiling. When he climbed the ladder and looked, he saw, at the center of the maze, the word "Yes." That's when he knew that he had to meet her. I've thought about that story a lot. That one little word was so intriguing because it means more than acquiesence or agreement. It implies open-ness, an open-ness to possibilities, a willingness to accept things rather than deny them or close yourself off from them. It is the opposite of no which implies intractability, refusal, digging-in.
I think that people often say no more easily than yes. Perhaps yes makes people feel vulnerable, as if they are agreeing to something they don't want or will regret later. No makes them feel more in control, but I've found that people who find themselves saying no much of the time are no safer; in fact they risk losing experiences, relationships, and a certain amount of courage that it takes to enjoy life. So before saying no, think about whether you are merely disagreeing or whether you are cutting yourself off from an opportunity that could bring you growth. Then consider yes, acceptance, and see how you feel.