Why does the UK have to pay the EU divorce bill?

Press quote (International Business Times)
28 March 2017

Late last year, it was thought that the European Commission was seeking an exit bill of €60bn – around £50bn. However, in February, the Centre for European Reform (CER) estimated that the bill could range from €25-73bn.

...The CER calculates what we can expect to pay based on what we owe, and what we can offset. It estimates something between €25bn (based on honouring commitments made in annual budgets and maximising UK receipts), and €73bn (maximum obligations, minimum receipts).

...According to the CER, only 25-30% of the biggest cohesion fund payments will actually have been spent by the time Britain is expected to leave the EU in 2019.

...The CER argues that Britain's exit obligations should be partly offset by its share of EU assets, rebates and budget receipts. In the EU's accounts, the EU's total assets amount to €8.6bn of property, plant and equipment, and €13.9bn of assets available for sale.