The adjective acquiescent describes someone who willingly carries out the wishes of others. If you say "yes" to every request for help, you may have an acquiescent personality — or the word "no" just may not be in your vocabulary.
Acquiescent comes from the Latin word acquiescere, which means to remain at rest or become quiet. The root quies is also the origin of the English word "quiet." Acquiescent people are sometimes thought of as quiet and mousy because they tend to do what you ask without protest. Try not to take advantage of these people by saying things like "Give me the last piece of cake," or "Do you mind walking my dog?"