Regulus Black tells himself that what he's feeling in his heart is nothing. He whispers to himself that this emotion called love is meaningless and worthless. The way his heart pounds and pounds frantically against his rib cage is nerve wracking; it's like his heart is trying to escape from him. Regulus is afraid that once he loosens the tight hold on his heart that it would no longer be his. He's afraid of how his eyes seem to follow the other man. He's afraid of breaking his heart.
He's afraid of the spell James Potter has cast on him.