Historically, all languages and communication strategies developed/evolved without FIML. Thus communication all but everywhere and with all but everyone relies entirely on non-FIML strategies. As languages evolved/developed, non-FIML strategies proliferated, in many ways making it more difficult for FIML strategies to arise. People became accustomed to non-FIML communication strategies, learned to enjoy them, and wanted to perpetuate them. This is still the case today. People are so used to non-FIML strategies, they cannot imagine anything else.
In many ways, it is because people understand only non-FIML strategies that they communicate to their loved ones by using public semiotics rather than clear interpersonal speech. They buy things, go on trips, go out to dinner, achieve status, and so on as substitutes for real interpersonal communication. Lack of FIML is also an important reason why many people enjoy their professional or public lives more than their private ones. Many people do like to bowl, but many of those same people also join bowling leagues because their private lives are unsatisfying.
If you do not do FIML practice with your primary interlocutor, you will be neurotic. Sounds bold to say that, but how could it be otherwise? Without FIML, you will not have clear communication with your primary interlocutor (spouse, SO, best friend, etc.); and without clear communication you will be forced to imagine what they mean and you will make mistakes and the mistakes will compound. In a short time, you will have a mistake-riddled, self-centered understanding of your primary interlocutor rather than a clear understanding of them. And the same will be true for them. This is why so many very loving, very compatible couples have problems within a few years.
Do you want to have clear communication with your SO? Do they want to have it with you? If you don’t do FIML, how can you get it? I don’t think you can. Do you think you can rely on feelings? On love? On good will between you? How will you prevent misunderstandings from developing if you have no way of knowing with great clarity what you are saying to each other? What is your strategy? Do you have one?
Make an arbitrary list of, say, ten words. Ask your SO to free-associate on each word; just have them say what first pops into their mind when you read each of the words. Have them do the same for you. Is it not clear that your associations are not the same? Maybe a few of them are, but most will be different. Now what happens when you speak sentences to each other? Can you see that you are always going to be making assumptions about what your SO is saying based on your own self-generated associations? How can you be sure you know what is in their mind when a certain tone of voice issues forth? Can you be certain you know why they chose that word or that phrasing? Of course you cannot be sure.
You can only know with clarity what your SO is saying if you ask them. But if you are only accustomed to non-FIML strategies, you will find that hard to do. As mentioned, communication all but everywhere and with all but everyone relies entirely on non-FIML strategies.
A wonderful result of FIML practice is it removes the need to wonder whether your partner has been bothered by something you did or said. This result occurs because you will gradually become confident that your partner will say something if they feel bothered.
Best of all, they will say something right away before whatever it is grows into something large and unmanageable. For example, if you use a way of saying good-bye that makes them feel lonely, they will bring it up right away and you can figure out what the cause was and/or how to do things differently if need be. This is a much better way to deal with something like that than for them to wait months or years before telling you, if they ever do. Imagine how just that one bad (also wrong) feeling might grow in them over time and lead to negative thoughts and actions that could have been avoided.
The best way to learn FIML is to break the practice down into small skills. Try the word list described above with your partner. Or start by just pointing to things. After you have had some practice, ask how some phrases make them feel or what they associate with them. Doing that will be fun and it will help you develop the skills needed for FIML practice. Stop, ask, hold your emotions in abeyance, listen, think. It gradually will become second-nature.