The common statement I make to a lot of my clients is, "No sarcasm, putting down others, even if they behave so. For your own good and growth, be kind AND assertive!"
What's assertiveness? To start with, being aggressive is not being assertive. Assertiveness is when one respects their feelings and needs on par with others'.
Thinking less of others' needs and feelings, and more of their own comes from arrogance, aggression, lack of consideration and plainly put, selfishness.
Thinking less of one's needs and feelings is timidity, which comes from a lack of self-confidence, self - esteem and plainly put, fear. And all of these, one can easily portray as martyrdom, sacrifice and selflessness - which is not good for them.
Aggression or timidity is not at all useful when practised too much and without thinking. It's important to treat oneself on par with others. Why? That's the truth - each and every being is equal. None superior, none inferior to another. Assertiveness means a well thought through behaviour. And most often means being soft-spoken, yet firm. To be this way requires awareness, considering multiple perspectives, confidence and compassion. Being kind equally to oneself and others, that's assertiveness.
Introspect on your close relationships to check how those relationships are. Become aware. Make appropriate changes. Take responsibility. Help loved ones be assertive if they are not. Become assertive if you are not. Ensure that the relationship is great. Remember, you are a constant in all your relationships.