They become resentful, but still have difficulty expressing the way they feel to others. They keep giving while others keep taking from them. Assertiveness training reduces stress and improves communication skills. Being assertive means being effective in any situation, be it managerial, parental or socially. It certainly affects the way others react to you.
Submissive people tend to ramble on and are loath to get to the point of a conversation.
There has been a great increase in the desire to learn how to communicate more effectively in recent times and there are many courses available and numerous books on self-assertiveness. There are, however, ways you can instantly improve your communication skills right away by Asking for what you want, but not being upset if you don't get it. Being more decisive and ready to cope with consequences of your own actions. Expressing your opinions and feelings without fear. Not being afraid of making mistakes.