Well you don't _have_ to grow fast. But a slow-growing or not-growing company also has a very different feel. Fast-growing companies have lots of room for growth for everyone who works there, because there are new positions and new responsibilities opening up all the time. Slow-growing companies have less of that, so somebody else has to leave or die before you get a chance to move up. Sometimes, this is still better than the problems of fast growth, but it is a tradeoff.

Also, if you took VC money to start the company in the first place, then you may require fast growth in order to offer some prospect of a return to the people who invested with you. Similarly, if you maxed out your credit cards and took loans from family to get going, you need/want to grow fast to pay that back. Not every small company is in that situation, but many of them are.