So, you will need to learn more about assertiveness before you can start dwelling upon this issue. Assertive behavior presupposes that you know what you want. In other words, you ask for what you want but you do not necessarily get it all the time. You know your rights but you also respect others which is why you do not become aggressive.
It is important to differentiate assertive and aggressive behavior. The latter means that a person does whatever they want. When the person is aggressive, they simply take what they need regardless of what others think. Being assertive means that you are confident. Yet, you are not boastful about it. Being assertive and being aggressive are two completely different things.
Another aspect to take into account in this respect is that the ability to be assertive might be crucial for some careers. Thus, your boss might actually respect you even more for having that skill which will clearly help you succeed in the field which you are working in.