There are a bunch of reasons for this. I'm sure this is in no way an exhaustive list.

• Most countries still have growing populations. So if the GDP isn't growing then GDP per person is falling.

• Lots of people clearly don't have enough stuff. There are people in the UK going hungry or cold because they don't have enough money. Increasing GDP can offer a way of making things better for these people without making anyone else worse off.

• Even people who clearly do have loads of stuff tend to want more.

• Businesses usually expect to grow, and a larger economy generally makes this easier.

• Many plans for the economy and government finances assume economic growth. For example plans for paying government debt or pensions are projected many years into the future, and assume the economy will grow. If it doesn't then paying for these becomes much harder. Not only that, but the impact will be felt straight away, eg. by interest rates on government debt going up.

• Countries competitive with others about some things - if the UK's economy shrinks there's less money to spend on its armed forces, for example.

• On a more cooperative note, more GDP means more money to spend on international aid, for example.