The Spirit must whisper.  The reason is simple.The Spirit speaks softly.  It speaks through our emotions.  In my experience, the Holy Ghost often speaks at the edge of my ability to hear.  It sometimes speaks in emotions so subtly that I can mistake my own emotions and desires for those of the Holy Ghost.

Its frustrating.  But also necessary.

The Holy Ghost is not primarily an Answered.  It is primarily a Comforter and a Sanctifier.

Emotions of comfort and peace do no good unless we feel them.  Knowing that someone else is at peace does nothing for our own souls unless we are good at empathy, which means that we ourselves feel the same emotion.  For the Holy Ghost to truly comfort us, it must share peace and comfort with us in a way that we ourselves feel the peace and comfort as our own emotions.

Sanctification requires having sentiments and reactions that are holier and purer than our own.  Somehow experiencing alien emotions would not do it.  We must experience them as our own.  We  must, in communion with the Holy Ghost, enter into the inner life of deity.

For both of these reasons, we cannot have an impermeable barrier between our emotions and that of the Holy Ghost.  Our own feelings and thoughts must blend imperceptibly into His.

And so we will not always be able to clearly tell our thoughts and our desires from His.