Loss Aversion
We prefer to avoid loss rather than achieving gain. We will protect what we have to a great extent than we will strive to acquire an equivalent amount. Example: we will demand more money for something we own than we would pay for that thing. Additionally, losing a thing is hard, in the sense that we rather to do something – to add something. We would rather add exercise to lose weight, rather than just subtract food.
Examples
A study gave participants a (theoretical) base car, and asked them to add options to it, in exchange for money (so, air conditioning cost $X, cruise control cost $Y, etc.). Participants generally outfitted the car to a certain level, and then decided further options were not worth the money. The study then gave other participants a fully-loaded car and asked them to remove options to save money (so, deleting air conditioning saved them $X, etc.). In general, the second group ended up with a more expensive car, because we are more motivated to keep something we “own” than we are to obtain something we have never owned.