As a new vehicle is driven, it's value decreases. When you lease a vehicle, the monthly rentals are based on how much the vehicle depreciates in value. For example, driving a vehicle 8000 miles a year will uphold it's value more than driving a vehicle 20000 miles a year. As such, increasing your annual mileage will increase the monthly rental.
There are other factors that can affect how much a vehicle depreciates, such as contract length and whether or not the vehicle is maintained by the funder.