Why did Elros become a man?

Could you please explain the reasoning behind Elros's transformation into a man? What specific events or factors influenced this decision? Was it a personal choice, or was it influenced by external forces? Could you elaborate on the significance of this transformation in the context of the larger narrative? Additionally, how does this transformation affect Elros's role and relationships within the story? I'm curious to understand the deeper meanings and implications behind this pivotal moment in Elros's journey.