My personal headcanon is that Rin understands love as being cared for, receiving attention based on his needs, being protected, and, why not, being spoiled.
Most people learn what love is during childhood—how it feels, what it looks like, how it's expressed, and how to receive it. How to love and be loved. For Rin, his first model of love was Sae; it was being the little brother.
That doesn't mean Rin is looking for an older brother, but rather that this is the way he understands love. It's much easier for him to recognize that someone loves him if they know his favorite food, pay attention to his moods, learn his boundaries, and stand up for him, even when it's not necessary. Rin would identify those things as love far more easily than a romantic confession, because they're familiar, and his mind interprets them as safe.
The same would apply to Sae, but in reverse. The way he shows affection mirrors how he used to treat Rin—not exactly the same, because his partner isn't his little brother, but the foundation is similar.










