Commentators explain that Elijah's despair wasn't a sudden loss of faith, but the result of extreme physical and emotional exhaustion. After the spiritual high on Mount Carmel, he faced a death threat, fled for his life, and was now alone and weary. His prayer to die shows that even the greatest servants of God are human and have limits. As Matthew Henry notes, God allowed this to show that Elijah's previous strength came from God, not from himself.