Successful restaurant owners see complaints as gifts. For every complaint you receive, there are likely many other guests who had something to complain about but decided to stay silent, leaving disappointed with no plans of ever returning.
Look at each complaint as honest insight or feedback on the way your restaurant is being operated and where you need to improve. You would normally need to pay a consultant for that type of valuable information. Additionally, each complaint that is handled well can turn a bad experience into a positive one if the customer is impressed by the competence and positive attitude of your team members. This is the type of service that generates loyalty and great social media reviews.
The key is to properly train your staff. Train your team to respond to complaints in a positive way, not in a defensive or adversarial way. They should immediately look for ways to resolve complaints in a way that will satisfy the customer. Reinforce the training with regular reminders that mistakes and complaints happen, and they should be seen as opportunities to create memorable moments for guests.
As an owner or manager, you can help by enabling and trusting your team members by giving them the authority to resolve guest complaints. You will end up with a more capable team who has the confidence to make decisions that lead to memorable guest experiences.