From Halopedia, the Halo wiki
The father of Michael Sullivan and his two younger brothers, Steven was married to Evelyn Sullivan. Steven Sullivan was a researcher for the UNSC's Office of Naval Intelligence and his job often prevented him from visiting his family. As a result, his children were mostly raised by his wife.
Steven insisted that Michael attend a military school, as he felt his son needed to learn proper discipline. Evelyn disagreed as it would mean sending her son away from home. Nonetheless, Michael eventually enrolled at the Corbulo Academy of Military Science on Circinius IV in 2526. Michael later desired to follow in his father's footsteps and become a researcher for the UNSC, rather than serve in a combat-oriented position.