Dara is Jahanara's oldest brother, and he's the presumed heir to the Peacock Throne. He's kind, and gentle, and he's a philosopher to the bone. These are wonderful qualities for a future leader, but they're problematic when they're not coupled with firmness and resolution during times of adversity.
In fact, Dara is so one-dimensional in his obsession with his studies that it just about single-handedly costs him his throne and his life.
While he was busy reading about Hinduism, for example, Aurangzeb made friends and allies in the court—and gained military prowess, as well. How can a guy buried in his books compete with someone like that? He's the Belle to Aurangzeb's Scar. Add in a healthy dose of sibling rivalry, not to mention Aurangzeb's intense animosity, and Dara is bound for trouble.
"I sought above all…to bring our people together," [Dara] lamented. Between the pauses in his words the rain roared, assaulting the fixed umbrella above him and falling onto his elephant. "I wanted Hindus and Muslims to live as one. I learned of religion instead of war because I thought understanding and respect would bring us together. I lived my life, dedicated my life, to achieving that union. Yet how I failed. For now I must fight my brother. And if brothers can't respect each other, how can strangers?" (16.53)
Despite Jahanara's efforts to rouse Dara to defend himself against Aurangzeb's treachery, it's no use. His evil brother publicly decapitates him, and his head is presented to their father in one of Aurangzeb's nastiest gestures. Poor guy.