Just because you agree with someone does not mean you're nice. It is a misconception that most people encounter on an almost daily basis. I am one of them. When someone agrees with me I usually, at least for a short period of time, find them to be great people. A person who disagrees with me or criticizes me, I dislike at first.

I always got the highest grades in Swedish. It has been my subject; the thing I am passionate about. Reading, writing and getting involved like a madman. And so I started high school and met Anders, my new Swedish teacher. He started by just barely passing me. A D-. I was completely upset. And he continued. D and D. Four or five times. In the end I stormed into his office demanding an explanation. He said that my submissions were great and I was doing fantastically well on every test. I wondered why the hell I wasn't getting an A instead of just barely passing. He thought that I could strive for more. I was lazy. He wanted me to stand on my toes to get the grade I wanted. I slammed the door and walked away. I was furious. The remaining three years in high school I did everything to get an A in Swedish. I worked my ass off. Struggled more than I ever have for a grade in Swedish. When I graduated, I got A in all his courses. It was his plan all along. 

Just because you disagree with someone does not mean you're not nice. Quite the opposite. If you want a person's best you should also challenge the person in question. It can be done on a daily basis by asking questions: Why and how? By challenging them in a constructive way. You can practice by getting into debates and discussion forums on the web. Decide to be a bad cop, the devil's advocate. Bite and be tough.