Fallout New Vegas is themed around letting go. Of what varies. Elijah can't let go of Old World tech, and Dean of the Sierra Madre, Vera, and Sinclair, in Dead Money. Joshua Graham can't let go of obtaining vengeance on the White Legs, and the Legion by association, in Honest Hearts. The Think Tank can't let go of the fact that there is no Old World for their SCIENCE to affect in Old World Blues. Ulysses can't let go of the past in Lonesome Road. The game's events came to happen because the NCR, Legion, and House can't let go of Vegas and Hoover Dam. Fallout New Vegas was caused by refusal to let go. Think about Westerns for a moment. Why would so many people move out west (east in this case) for something they may not find? It's simple. Greed. Elijah and Dean want the Madre's famous gold stash. Josh wants his own sense of revenge fulfilled. Big MT's scientists wish for their creations to be used by their directives. Ulysses wants The Courier to experience what he felt years before. The major factions want the Dam's power generation capabilities to themselves(except possibly the Legion). That greed leads the NCR to eat losses in the Mojave despite obvious local objection, the Legion to throw slaves and warriors into the breach, House to plot betrayal, Elijah to experiment with the bomb collars, so on and therefore. But the Wild Card represents letting go. You chose not to follow any but yourself. You can choose to leave the treasure of the Madre. You can talk down Graham from executing Salt-Upon-Wounds. You can convince the Think Tank to your side. And you can stop Ulysses from possibly destroying any hope of civilization rebuilding. In short, you've let go.