Verbal communication is a skill, one must be careful about how you say things to avoid misunderstandings because everyone has different meaning for same words. In this world what you say doesn’t matter as much as how you say it. When putting forward an opinion, you need to think about your choice of words, your gestures, body language and about your tone intensity pitch. 

My father always used to say, “It’s not what you say, it’s the way that you say it!” I think that this is correct and incorrect at the same time, as it's both what you say AND how you say it. The non-verbal components of an assertive message are really the key to its effectiveness. When a customer sees you - your look and your body language - you are already communicating to them, non-verbally, and they are deciding whether or not they want to listen.