Seto's world revolved around fighting. This meant he was likely to die young if he ended up fighting the wrong enemy. At a young age he ended up in Soul Society. This world was different. Nobody wanted to fight. Nobody worried about hunger either... but Seto did. He walked around until finally his hunger became too much.
Ukitake wandered around the Rukongai, frowning as he found the child's body up against a wall. The child had obviously died of starvation. That fact meant that he had a vast spiritual energy. It was another child that wasn't rescued by the academy.