In today's Gospel, Peter shows both his impulsiveness and his inconstancy - two characteristics that make him easy to relate to.
It's around 3 o'clock in the morning as the Apostles battle against a stormy sea, and Jesus comes walking across the lake towards the boat. The Apostles are scared stiff - they think they are seeing a ghost.
Even Jesus' reassurance doesn't allay their fears, so Peter takes the matter into his own hands and challenges the ghost to do something that only Christ could do - enable him to walk on the stormy water. And he does - for a few steps. But then Peter takes his eyes off Christ; he looks around at the waves and the storm, and he starts to sink.
As long as Peter kept his eyes on Christ, he was able to walk unhindered through the stormy sea. As soon as he let his eyes wander away from Christ to examine the intimidating waves, he began to sink.
Just so, as we strive to make our way through the stormy temptations and challenges of life in a fallen world, only focusing on Christ can keep us afloat.
Christ is always close to us in our storms, asking us to believe in him.
In his words to Peter, tinged with disappointment, we see how much he longs for us to trust him: "Why did you doubt?"
As soon as Jesus steps into the boat, the storm gives way to peace and calm. Christ wants to be our peace, our strength, and the solution to life's troubles.
St Peter didn't learn this lesson right away, but he learned it well; in his First Letter he put it like this: "cast all your anxieties on him [Christ], for he cares about you" (1 Peter 5:7).
As a closing I like to quote word from St. Augustine:
"If I try by myself to swim across the ocean of this world, the waves will certainly engulf me. In order to survive I must climb aboard a ship made of wood; this wood is the Cross of Christ. Of course, even on board the ship there will be dangerous tempests and perils from the sea of this world. But God will help me remain on board the ship and arrive safely at the harbor of eternal life."