So in the show, Foggy says something about how Matt must've kept himself ready to fight long before he beat up that pedophile guy. This is somewhat inspired by that.
In undergrad (or the final year of highschool, as long as Matt is 18+), years after Stick left and he stopped regularly training to fight, Matt is raped by an OMC. Afterwards, he's consumed by guilt/shame/just some horrible feeling that he should've been able to fight his rapist off and it's somehow his fault that it happened because he didn't defend himself.
It's after that, that he decides to take up fighting again. Every time he has another flashback, or wakes up from another nightmare, or runs into a stranger who reminds him even a little about what happened... he goes to the gym, and he pushes himself just that much harder.
Young!Matt, noncon, trauma as 'motivation'
In undergrad (or the final year of highschool, as long as Matt is 18+), years after Stick left and he stopped regularly training to fight, Matt is raped by an OMC. Afterwards, he's consumed by guilt/shame/just some horrible feeling that he should've been able to fight his rapist off and it's somehow his fault that it happened because he didn't defend himself.
It's after that, that he decides to take up fighting again. Every time he has another flashback, or wakes up from another nightmare, or runs into a stranger who reminds him even a little about what happened... he goes to the gym, and he pushes himself just that much harder.