“I’m not holding back my fire out of spite Midoriya, I literally can’t use my left side since my quirk is dependent on my emotions.” Shoto explained to Izuku.
Originally Izuku believed that Shoto held back his fire, but Shoto told Izuku about his past and his situation.
Turns out Shoto’s Quirk operates as a strict somatic mirror of his autonomic nervous system. The right side is governed by calmness, stoicism, and in general being like ice itself for him to use it. The Todoroki estate turned emotional detachment into a survival instinct, making his ice effortlessly lethal; whenever Shoto feels threatened, isolated, or detached, the frost expands instinctively.
The left side, however, is wired directly to sympathetic nervous activation: high emotional states like fury, grief, frantic desperation, or unfiltered passion. Igniting requires a sudden spike in adrenaline and emotional vulnerability.
Because childhood abuse conditioned Shoto to shut down his feelings the moment tension escalated, it meant Shoto can’t ever use his fire even if he tried.
He only used his fire against Midoriya thanks to feeling mild frustration trying to get that headband.
To use both sides Shoto feels that he must go crazy, since he can’t stand constantly changing emotions like he’s borderline or bipolar just to use one side over the other instantly.