admitting is a sign of great courage
At age 12, Hiro, the Sheep, and Kisa, the Tiger, are the youngest among the current zodiac spirits. Hiro is snappy and verbose, always having a retort at hand to counter anything others may say; as a result, he comes across as older than he is. Behind that exterior, however, is a young boy who knows full well just how helpless he is in some situations — especially matters of the heart, with his affections belonging to Kisa.
Before the events in the story, Hiro had told Akito, the head of the Sohma clan, about the feelings he harbours for Kisa. Furious, Akito struck Kisa and inflicted enough damage for Kisa to be hospitalized. Deeply shocked and ashamed, without fully understanding the horrors Akito is capable of, Hiro was for once at a loss for words, and distanced himself from his childhood friend thereafter. Even when Kisa became the target of bullying at school later on, Hiro wouldn’t attempt to approach her. Instead, he grew more and more disgusted at himself for claiming to love her nevertheless.
After Kisa is “saved” by Tohru, Hiro confronts Tohru multiple times out of jealousy because she managed to do what he hadn’t been capable of. At one point, he means to mock her about her ability to comfort seemingly anyone, but his mockery turns into an admission of his own inadequacy and cowardice: “Why…? Why am I such a child? I ought to know better! […] (I hate myself. I’d rather die than remain as the child that I am… I don’t want to be like this! I want to finally become a proper adult…)”
Seeing Hiro struggle with himself, Tohru lets him know that she thinks he’s great — especially for his age.
My mother always said that it’s difficult to admit to yourself that you’re still a child. Most people are far too afraid to do so. That’s why admitting that to yourself is a sign of great courage. Tohru, Volume 7
Tohru adds that she firmly believes that Hiro will one day become a prince capable of protecting Kisa due to his courage — more specifically, the courage and strength not to give up.
True enough, these words allow Hiro to believe in himself again, and to try to do his best in his current position.