Think of it in terms of potential energy. The amount of energy you have is based on how much fuel you have, how heavy your craft is, and how efficient the engines are in converting that energy to motion. How much energy you need to accomplish what you want is mostly based on the gravity of whatever system you happen to be in (and atmospheric drag if you're in atmosphere). In a ship with no other forces acting on it (gravity or drag), the amount of dV that you have is how much your velocity will change if you use it all. Gravity and drag subtract from that total.