Do people take advatage of your politeness?

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I have met some people who take advantage of sincerity and politeness. Since i m sincere as much as i can in my duties in my office, people are now dependent on me that i will be there to take a lead. And I have never quarelled with any other seriously and i never use a bad or harsh words aginst anybody whereas people donot understand that. Should I change my attitude or what the other way to deal the people in better and right way? Am I ridiculous when I feel that I m used?pliz suggest.