"Assertiveness is a skill regularly referred to in social and communication skills training. Being assertive means being able to stand up for your own or other people's rights in a calm and positive way, without being either aggressive, or passively accepting 'wrong'." --skillsyouneed.com

I strive to be assertive and kind towards others at the same time. I think this is the definition of assertiveness. If you are saying what you think, and being hurtful at the same time, thats not assertiveness. On the other hand, it is of course possible to hurt others feelings unintentionally, while being true to yourself. But I think...if people are going to be unhappy with you because you put a boundary up, or say what you need, or correct them, the more reason to enforce that boundary, and even steer clear of them because that is called "being a jerk." In this instance, I am not afraid of being hated for saying how I feel. It is scary to me though, and having someone say to me: "heads up, people may hate you if you say what you really think, it happened to me," is not helpful to me.