Question: why did aemon become a maester?
Answer: His grandfather, Daeron II, recognized that too many Targaryens was "as dangerous as too few," and sent Aemon to the Citadel to become a maester when he was an older child. Aemon forged his chain as a maester, despite his father Maekar's disdain for it (the idea of a Targaryen "serving" anyone else irked him).