Drowning the PainShinjiro became like an older brother to Akihiko and his younger sister, Miki, during their time in the orphanage. He really cared for both of them and was completely devastated when Miki was killed in a fire. Akihiko was a disaster, however, so Shinjiro pushed his own feelings aside in order to support his friend. He never had a chance to grieve himself.
On an early SEES mission, Shinjiro loses control of his Persona. Castor goes berserk and destroys the surrounding area. Unfortunately, the destruction resulted in a casualty of an innocent woman in the vicinity--Ken's mother. Knowing he was to blame for killing a mother and destroying their house was too much for Shinjiro to bear. He knew all to well what it was like to lose a loved one and struggled to come to terms with forcing those horrible feelings on someone else. Unable to find an emotional resolution, Shinjiro quits SEES and begins to do get involved with the shady Strega in order to get Persona suppressants. He hoped by throwing everything away and repressing his power, he could forget about everything that happened and the immense pain that came along with it.
Shinjiro has dealt with highly traumatizing events throughout his life. Though there is no mention of his real parents, his early life was spent in an orphanage. Something may have happened to his parents or perhaps he never knew them. Either option could be a source of some pain. After finally building family ties with two other children there, he loses one of them to a fire to which he feels partially responsible. Further, he wasn't allowed time to work through his feelings of guilt, pain and despair, because he took it upon himself to take care of Akihiko. Accidentally killing Ken's mother was the worst event of them all, however, because he could not deny he was directly responsible. Even though he certainly did not take her life on purpose, she died because he was there and could not control his Persona. Not only did Shinjiro have to deal with the terrible guilt and remorse of taking a life, but he was also reminded of the immense grief he never had a chance to deal with from the past. Would he still have chosen the same path if he had been able to work through his grief over Miki? Probably. But perhaps he wouldn't have completely lost his footing in the flood of painful emotions. Personally, I believe Shinjiro became too overwhelmed with powerful emotion because of all the trauma he's experienced and had to drown the pain away as a defense mechanism. He simply could not handle it. Running away is much easier than working through painful memories and feelings, after all.
While I think most of Shinjiro's actions from the day he left SEES to his final moments in the game were fueled by the desire to atone for the past, I also believe his unwillingness to face his pain is also a powerful motivator as well. Not only does he push Akihiko and Mitsuru away because he truly feels he no longer deserves to be happy, but being with them would only be a unpleasant reminder of both Miki and Ken's mother. Being a member of SEES and using his Persona would only bring back images of Castor going out of control. Arguably, Shinjiro would also have a difficult time shutting Akihiko and Mitsuru out of his heart because he cares for them. Being around them would make him feel and he couldn't bear feeling anything but numb. In a similar vain, changing his lifestyle completely could have also been an attempt to avoid any reminder of who he used to be. If he didn't go to school, didn't live at the dorm and didn't spend time with his friends, he wouldn't be as reminded of his past. It would be easier to repress the feelings and memories in unfamiliar settings. Ultimately though, becoming numb goes hand in hand with his strong desire to atone for the past and it is difficult to discern if one caused an action more than the other.