There are many conditions that can cause one to loose assertiveness skills once they have been maintained for many years. One thing I discovered, in hind sight, when I was a very assertive person, I was less tolerant of those who attempted aggressive behavior, manipulation, and games. They many not have known they were even doing it. If it was someone such as one of my kids, I communicated assertively. There are some unreasonable relatives that I just chose not to have much communication with, as it was work.
I know what it feels like to be assertive and I can only say that it is wonderful. Every day was a joy. I didnt always get my way by far, and it was not a life without pain, but I had a voice, was reasonable and honest, and lead to good esteem. I rarely regretted anything I said. I had a traumatic brain injury from a fall, which led to a sequel -depression, anxiety, severe fatigue, which in turn led to frustration in communication, allowing people in my life that were very unreasonable, leading to more poor communication, frustration, anger, feelings of loss of power and choices, etc. I realized this and was working on my communication skills when I was assaulted by a police officer and abused harshly. This left me with loss of use of my arm, but it took my voice away(figuratively). It taught me to never assert myself again, as it can be dangerous. I can see how this has been self destructive and left me not wanting to live. Its not that I really dont want to live, I just dont want to live without any rights and walking on eggshells.
It has been 4 yrs since the assault and I am just trying to regain my assertiveness skills. This is important in or out of intimate relationships. We are constantly communicating with others, friends, co workers, bosses, sibling, parent, child, neighbor, store clerk etc. When we stuff things, we do eventually blow, or it comes out in addictions or other unhealthy behavior.
I can attest that when assertive, I saw life in a completely different way, different perception. It kind of ironic, I taught assertiveness to battered women-and now cant speak for myself. Just an opinion, but I do think it is key to our thinking. If we always feel defeated-why try.