I have found that meaninglessness leads to deep anxiety and confusion. I'll use family relationships as an example. If you do what you want and assign your own meaning to your actions and choices, those around you will probably not see the same meaning in your actions. To be more specific, if you think "I can flirt with this woman at work. It's fine. It's harmless." Things can quickly escalate and either your spouse will soon feel abandoned, or you will lose your job or worse, you will develop a relationship with someone besides your spouse and live a compartmentalized life. This, eventually, will come to an end and many lives will be torn up as a result of you saying "I can do what I want."

"I can do what I want and make it mean whatever I want" is a very selfish delusion that completely ignores that your actions affect those around you (who are also assigning meaning to your actions and words) and that those around you affect you as well.

Another danger to this is the delusion that you always know what is best for you. Many times in my life, I have gotten advice from someone who was watching me and who told me I was being reckless or foolish, and that I should think about what I was doing carefully and choose another path. I could have easily dismissed them (and often did), but sometimes those outside of our head can see what we are doing or where we are heading more clearly than we can with all of our justifications.

In short, meaninglessness means that at any time you and anyone around you can decide that marriage doesn't mean commitment anymore. You can decide that your spouse cheating on you is a good thing. You can look forward to feeling betrayed. And hopefully your children will just be able to change their minds about how their parents feel about each other. And hopefully they don't change their minds about how they feel about you. It's all fine because you can just make it mean whatever you want!