Disclaimer: Don't own Fringe. Unfortunately.
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Everyone in this world has a feeling, which makes him feel at place. Not necessarily happy, but powerful, proper. Like a puzzle piece in its place.
For Walter Bishop it was a feeling of discovering new, unusual phenomenons.
For agent Broyles it was closing a successfully completed case.
For Peter Bishop it was his freedom, the ability to make his own choices.
Finally, for Olivia it was the thrill of holding a loaded gun in her hand, knowing that in that very moment she was the master of the situation.
Everyone in this world has a feeling, which makes them feel at place. For ones it's usual for others even unthinkable. But the important thing is - it helps them survive.