“You of little faith,” he said, “why did you doubt?” (Matthew 14:31)

Not small faith as in size but little faith as in quantity. It's not a question but a statement of measure from the results of Peter walking on the water. What's the leading indicator? Doubt. Peter was obedient to "Take courage" (Matthew 14:27) and to ask for confirming command (Matthew 14:28). When hearing the command he obeyed against all logic and came toward Jesus on the water. 

Why did he doubt? He saw the "strength of the wind." (Matthew 14:30)

• strength: ἰσχυρὸν (ischyron) Strong's 2478: Strong (originally and generally of physical strength); mighty, powerful, vehement, sure. From ischus; forcible.
• of the wind, ἄνεμον (anemon) Strong's 417: The wind; fig: applied to empty doctrines. From the base of aer; wind; by implication, quarters.

Not like "The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit." John 3:8

• wind πνεῦμα (pneuma) Strong's 4151: Wind, breath, spirit.
• blows πνεῖ (pnei) Strong's 4154: To blow, breathe, as the wind. A primary word; to breathe hard, i.e. Breeze.

 Lord, help me to have much faith as I follow where you call and trust in your guidance.