Because knowledge is always just a progressive noisy estimate of reality, which is infinite and unknowable, knowing something is “true” really just means believing something is true. Believing something is true requires that it: 1) is based on something you have observed, and 2) fully reconciles with everything else you have observed.
Very few people in this life live such that 1) all they believe fits these criteria, and 2) everything that fits these criteria is numbered among what they believe.
Many times, what we ask for is merely what we wish was true. Seeking what you wish without regard to what is real is what it means to lust: “...ask not, that ye may consume it on your lusts, but ask with a firmness unshaken, that ye will yield to no temptation, but that ye will serve the true and living God.” (Mormon 9:28)
Faith is not wishing. It does not mean hoping in whatever thing you wish was true. You have to know it is true.
You can’t ask in service to the true and living God without asking what is true, and anything less than this is temptation and lust. You also can’t have a “firmness unshaken” unless what you seek is fully reconciled to what you know.
Willfully opposing your current best understanding of what is God’s will diminishes God’s power in you. Acting against what we know injects doubt into our minds. God’s power is like an electrical arc that connects him to us. You won’t get an arc on an insulated substance.
...when we undertake to cover our sins, or to gratify our pride, our vain ambition, or to exercise control or dominion or compulsion upon the souls of the children of men, in any degree of unrighteousness, behold, the heavens withdraw themselves; the Spirit of the Lord is grieved; and when it is withdrawn, Amen to the priesthood or the authority of that man. (D&C 121:37)
If you turn away from your sincere understanding of right and wrong, or turn away from full reconciliation to God’s will as you understand it in your acts, beliefs, desires, thoughts, etc., you lose his power. And you will also lose the truth you previously held:
Behold, ere he is aware, he is left unto himself, to kick against the pricks, to persecute the saints, and to fight against God. (D&C 121:38)
You can’t believe in something if it is contrary to your understanding of reality. You can’t believe in what you ask God if you don’t believe God wants what you ask to happen. And this is no matter of wishing. You have to believe he wants it to happen, and that belief must be based on something other than your desire. You have to know enough about him that you sincerely believe he desires it, not that you do.
If you do not have direction from God—if you do not know as clearly as the day from night—what is right and what is wrong, what will lead to improvement and what will lead to degradation—do not do anything but seek until God gives you more light. You have to find concrete reasons external to you that give you the unshakeable belief that you are seeking what God wants. If you don’t have these reasons, you need to study and pray until you either find them, or find reasons why what you are asking for is not his will. Note that whether by counterexample or lack of example, you can’t have faith in something without having a reason to believe it. Not only do you need a reason to believe it, that reason must be strong enough to overcome any reason you might have not to believe it.