Actually it stems from my childhood. In Digimon Frontier, the characters had a Human Spirit and a Beast Spirit that would allow them to become Digimon. The Beast Spirits were extremely difficult to control, and the Gogglehead (lead character) Takuya nearly killed his friends until the youngest character Tommy/Tomiki stepped in. By putting himself in harm's way, he helped Takuya control his Beast Spirit.

So, in a nutshell, the only way the brothers could learn to control their beast forms would be to find something that's worth protecting, be it a brother or lover.