If you think about it, the reasons for the inability to refuse to have historical roots. Before, when our grandmothers and mothers were children, to get into someone else's space, handing out advice right and left, raise their indications of other children to poke their nose into other people's business was considered the norm. For this reason, defending personal boundaries is true to this day. People are afraid to say "no" because they are afraid their failure to provoke a conflict. A year ago I had a few customers with this problem and their wording was something like this " I used to be good, and if I refuse, I become bad, I'll concede." This type of statements often found in life. In fact, conflicts are always manageable, clarifying its position by offering something in return. So maintain the balance in the relationship.
the Ability to deny a useful skill, and it can be learned.
Learn to refuse the boss is not less important! You need to know his powers and not to take on excess capacity; it is always possible to designate the leadership of his workload at this stage of the work; for failure to say "I'm a little busy, but later will return to this issue", "Now I got work to do, if I won't be able to do everything" or " what I refuse (what cases orders) to fulfill your order?" etc. The main thing to refuse reasonable and fair and then the failure does not cause doubts.
Keep your boundaries is an important duty of every human being.