What is the obsession and how does it work?

Obsession is defined as "an idea or thought that continually preoccupies or intrudes on a person's mind." An obsession is something that, try as they might, an individual can't separate themselves from—almost like an addiction. Regardless of negative consequences, whether it's something positive or not, or how rational it is, an obsession is something that is stuck or ingrained in a person's psyche.
In the novel, Bobby becomes obsessed with living up to the life Dr. Hudson essentially gave him. In our world, obsession works much the same way—something creeps into our mind and implants itself there, like Bobby when he is told not to squander the new chance he has and when he can't stop thinking about the fact that a man essentially died to save him. There is no logical conclusion to an obsession. If it's a question or a conspiracy theory, the mind tinkers away and continues to work at the problem long after it's been answered. Until the mind feels satisfied, the obsession will be a driving force that overcomes everything else—in some cases, overwhelming people's desire and ability to eat or sleep.