Explain Like I'm 5 (ELI5)

There are two levels at which economic growth can (and must) manifest. At the business, i.e. micro, level and at the societal, i.e. macro, level.

At the micro level, Marx already described the rationale guiding the individual Captialist's motivation to seek out growth: stagnation leads to losing out over the competition. If your competition manages to produce more X for the same price as you, they can sell it for less, resulting in your loss of market share and profits. Hence, you yourself are required to seek out technological innovations i.e. boosters of production quality, speed and efficiency, in order to outperform, or at least survive alongside, the competitors. The inherent logic of Capitalism itself nudges you this way rather than individual vices.

At the macro level, we are not looking at individual firms anymore, but at the financial/economic makeup of a capitalist country. Here, there are a lot of structures in place which necessitate a perpetually growing economy. For example, as per Keynes theory, a state of full employment can only be brought about if investment (i.e. the spending of money in return for more money in the future) takes places at a steady rate. That is because employers only hire new workers if they expect demand for the product they sell to rise in the near future (relating the full argument here would be too complex). If investment were to dry up, the economy would slowly grind to a halt and fall over like a bike, which in order to stay upright, requires forward movement.