If you were to identify the most important thing associated with your existence—what would it be? It seems it would have to be something that you could-not-possibly-function without. Someone special, a spouse or close friend may come to mind, a person that you may believe you “couldn't live without.” But to be sure, you could live without this person. It may not be easy, but your existence is not dependent on theirs. So it seems that a person, irrespective how important they may be to you, cannot in principle be the-most-important-thing-associated-with-your-existence.
Given a little time for reflection and introspection, most people will identify some variation of their mind as the most important thing associated with their existence. If you think about it, everything you have ever done or could possibly do, is inextricably dependent on the existence of your mind. One may object and say that the mind is equally dependent on the existence of the body, and that without a strong healthy body, a person cannot function properly. But it seems that the mind would still take precedence over the body. A person suffering from heart disease, for instance, can still exercise the power of their mind. They can make decisions, communicate, process information, express joy or sorrow. But a person without a mind cannot do this, even if their body is in perfect health. Naturally a vibrant healthy body is to be preferred over one which is failing, and though a healthy body serves to reinforce the mind, the mind doesn't seem to be dependent on the body in the same way that other things (such as intentionality) depend on it for existence.
Consider the analogy that the mind is the engine in a car. Everything a car is capable of doing is clearly dependent on the engine. Yes we can turn on the radio or the headlights without starting it, but by doing so the battery will be drained. It is the engine that (while running) keeps the battery charged. Yes we can put a car in neutral and drive it down a hill, but without an engine to further propel the car, it will eventually slow down and stop. The engine itself, however, requires other things, such as clean fuel and lubrication (oil), in order to run properly and effectively. It seems the same can be said of our mind.
As mentioned, everything we have ever done or could possibly do is inextricably dependent on the existence of our mind. But what does the mind itself require to “run” properly and effectively? Information. It is information that appears to be the “language” or “fuel” of the mind. Information provides the mind with the “raw materials” needed to think, to reason, to learn and develop. Without information, your mind has nothing to “work with,” so to speak. But just as dirty fuel or oil will cause the engine of a car to run poorly, a consistent diet of valueless information will cause your mind to lose capacity and adroitness. If a person ceases to learn, for instance, their mind will consume nothing but the information it contacts in daily life. And though some of this information may be of value, the majority of it is certainly not. Unless a person is actively committed to the consistent learning of something, their ability to reason and think will slowly be undermined.
Instead of watching that movie or TV show that you've seen a thousand times, find a topic of interest and begin exploring it. Rather than reading yet another fiction book, pick up something nonfiction. The things that comprise our world can be far more exhilarating than mere pop entertainment. Find a hobby, develop a skill, read, learn, grow—do something! But whatever you do, don't be content with merely being entertained. A byproduct of constant entertainment is boredom and boredom often leads to a loss of initiative and eventually depression. If your mind is your greatest asset, shouldn't you put something useful in it?