Desire answers the question of what you want. Reason tells you why you want it. Through reason, you enumerate the space of possibilities and estimate the value of each alternative. You collate and analyze the reasons to believe and disbelieve these possibilities, the putative paths you posit will deliver them, as well as the basis behind your estimate of their value.
Beliefs are not meant to be whimsical or based only on desire. Peter said:
...be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear: (1 Peter 3:15)
To the degree that a belief is not backed by reason, it will lead the holder to outcomes less beneficial than would otherwise have occurred, impede possible improvement, and prevent them from learning from mistakes.
God created humans to act—to intentionally design and choose coherent courses of action—not to be acted upon, or limited to whatever outcomes they are handed in the form of beliefs detached from any reason for holding them. Beliefs that are based on desire and not backed by rationality cause the holder to be acted upon. Desire without reason, by definition, is not an action of intent, but a passive reaction.
A lack of reason will cause a person to desire things that are ineffectual:
He that wanteth understanding will think upon vain things: and a foolish man erring imagineth follies. (Ecclesiasticus 16:23)
You can’t know what will improve the most without reasoning through your options. You must attempt to figure out the cost and the benefit of each course of action. While this sounds mechanical, it is not only an act of great faith, it is a key component of the faith cycle.
By applying reason to the experiences of others and to your own, you can come to a more accurate valuation of outcomes, and that valuation can motivate you to desire things of greater worth than what normal people desire.
Desire without reason is worse than worthless. Desire without reason is the fullness of the baseness of fallen man. When coupled and exercised in their fullness, desire and reason are the propellant that will lead a person to eternal life. Reason alone is insufficient to do so, but reason never exists alone. It will always generate desire, and, unlike the default desire of mankind, desire from reason will lead a person to improvement and greater good to the extent they practice it.
Reason is not passive or emotive. It is active, intentional, deliberate, and logical. Reason reveals truth, which is immutable and transcendent. The truth doesn’t care how you feel about it, nor whether you even correctly understand it. Truth simply exists. How much we will act so as to discover and live according to more of it is our choice.
Reason rests on the assumption that you value truth more than anything else. Without truth as your highest ideal, reason will always be merely some shade of rationalization. Rationalization starts with a fixed desire and finds reasons to justify that desire, typically while ignoring plausible reasons that contradict that desire.
Reason is not rationalization. Honest reasoning requires that you are willing to change your desire.
Reason in the faith cycle is concerned primarily with discovering reality—things as they really are—while desire deals primarily in how we wish reality to be. Wishes have their place, but that place is not in reason, except as far as they motivate us to act in ways we otherwise would not to find and follow the pre-existent paths in reality that lead to outcomes we desire. Even in this case, ruthless reconciliation to reality is required to successfully obtain what we seek. If you have reason to believe that some ideas, situations, or people will detract from your goals, you must contend with them. Simply pretending they don’t exist will prevent you from obtaining what you desire.
Reason will root you in reality. Reason will help you be honest. Honesty is not an arbitrary quality. Because honesty is a measure of alignment with truth so far as you understand it, and because the truth determines relationships of cause and effect, honesty is also a measure of how effectively you are working towards outcomes you desire. Any degree of dishonesty indicates a person is intentionally deviating from what they believe is necessary to obtain what they desire.
Desire serves well as a priming force for reason. Desire can impart momentum to action in a way that is difficult for most people to muster through reason alone.
Desire can’t give you certainty. Desire is a feeling , not a fact. Reason takes a person from desiring to believe to actually believing.
Desire is fickle. No matter how intense, desire can never be as powerful a force for enduring as reason can be. No matter how intense it is, a desire can be undone instantaneously and unpredictably through the arrival of another potentially arbitrary emotion. A desire’s power stems from its intensity, not its goodness or substance. While emotions can be exceedingly powerful, there is no requirement that they be based whatsoever in reality. For example, imagine the pain you would feel to hear a loved one died, even if the news was—unknown to you—untrue. The intensity of emotions is not based on the degree of accuracy of the information generating them. In contrast, you can’t have logical confidence beyond the strength of the underlying reasoning. A strong rational reason cannot be undone without either superior rational reasons or dishonesty. Reason locks into improvement and can only be honestly undone through finding something better.
All honest reason leads the individual closer to God. Every good thing imaginable here on earth—no matter what it is—will lead you closer to reasons to believe that seeking Jesus is the core foundation of everything that matters here, and to the only place where all good things are found and endure.