I feel you, it is repetitive. But at the same time, I dont think its necessarily for convenience or lazy writing or anything like that. Its showing that Dexters lifestyle always leads to loss. It always has, and it always will. For him to indulge in his urges, means that those around him that he cares about pay a price. And hell never find a way to balance it so that doesnt happen. He can either stop killing, or stop getting close to people, or keep watching them pay for getting too close to the BHB.