Devavrat was deeply devoted to his father and didn't want to cause him pain. To ensure his father's happiness, he willingly renounced his claim to the throne and took an oath of lifelong bachelorhood. His brothers were unwilling to make such a sacrifice, and thus, Devavrat made the difficult choice to remain unmarried and serve as a protector for his family and kingdom.
In recognition of his selfless act, the gods granted Devavrat the boon of choosing the time of his death. From that point on, he became known as Bhishma, the one who took a terrible oath, forever bound by his vow to uphold the dharma and serve as a revered elder in the Kuru kingdom.