How do we respond to the tug of God on our emotions?
Do we follow His lead obediently?
Do we try to grab the bit in our own teeth and rely on human counsel?
Do we allow others to rein us in?
All too often, people get their counsel from some other source than God (i.e., friends, family, television, radio, Internet, etc.). So, when they find themselves alone and in the darkest night of their trial they have no reference point for hearing the Word of God, or being sensitive to the tug of God's Spirit.
The psalmist was aware of his need for sensitivity to God. Especially when walking a dark and uncertain path. Learn to hear the voice of God and allow Him to gently lead you on the road of life.
I know that I'm mixing the two meanings of this one word, but I hope that you can relate to the emotional aspect of your reins and the comparison to the reins of a horse. Unless you allow God to control the emotions of your life there will be times when you run rampant and scared away from His will for your life.