What drives customers and users to spread the word about particular products and brands?
In my opinion there are 3 things that drive customers and users to spread the word to others.
Keep in mind that each of the 3 points mentioned above include both their positive and negative states. A satisfied customer may tell (on average) 1 - 10 people (friends, family, coworkers, etc). However, a dissatisfied customer will tell potentially 10 times as many people about their bad experience (bad product, low quality product). Keep in mind that the internet and social media have greatly increased an individual customer's reach and has reduced the time it takes for opinions and thoughts to travel from person to person and multiple locations.
There are always different triggers based on the audience you're talking to. At its core though, there is one top-level notion that, if you focus on, lets you "plan for word of mouth"
Be authentic, Have a conversation
There is so much choice in just about every market now that you need to distinguish on more than product and service. Your customers want to feel valued and respected. They want everything to be perfect, but moreover they want to know that if anything does go wrong that it will be resolved, and your business will do anything it can to rectify or remedy the situation.
As soon as you make your business approachable, your customers, potential customers, and the people they talk to, will see you as more than a drain on their bank account.