Why did Peter begin to sink while walking on the water?

Answer: He saw the wind and was afraid

When Peter saw the strength of the wind, he was afraid and began to sink. Jesus caught him: "You of little faith, why did you doubt?"

“But when he saw the strength of the wind, he was afraid and, beginning to sink, cried out, “Lord, save me!” Immediately Jesus reached out His hand and took hold of Peter. “You of little faith,” He said, “why did you doubt?””— Matthew 14:30-31 (BSB)