Adam Clarke on John 13:37

Why cannot I follow thee now? - Peter probably thought that our Lord intended to go some long journey, which would necessarily subject him to many inconveniences and fatigue; and he felt quite disposed to follow him in this supposed journey, at all hazards. He saw no reason, because he did not see our Lord's meaning, why he could not follow him now.

I will lay down my life for thy sake - Poor Peter! thou wast sincere, but thou didst not know thy own strength. Thou wast at this time willing to die, but when the time cams wast not able. Christ must first die for Peter, before Peter can die for him. Let no man think he can do any thing good, without the immediate assistance of God. Peter's denial should be an eternal warning to all self-confident persons: though there be sincerity and good will at the bottom, yet in the trial these cannot perform that office which belongs to the power of God. We should will, and then look to God for power to execute: without him we can do nothing.