Why did country folk says that platero was like a steel?

Country folk compared Platero, the donkey in Juan Ramón Jiménez's "Platero y yo," to steel due to his strength, resilience, and unwavering spirit. His robust nature allowed him to endure the challenges of rural life, embodying the toughness required to navigate the harsh realities of the countryside. This comparison also highlights the deep bond between the narrator and Platero, emphasizing the donkey's reliability and steadfastness as a companion.