What causes build up of earwax?

Earwax production and characteristics vary genetically. People who come from East Asia tend to have very dry earwax whereas people who come from the west tend to have genes that cause them to have wetter earwax. You tend to increase the production of earwax if it's got a job to do. So if you're in a very dusty, dirty environment or if you have infections locally, you make more earwax in order to soak up the dirt and prevent the bugs from creating more of an ingress. The downside is that the cerumen, the thick wax, can build up and block your ear canal. You sometimes have to soften it with something like olive oil so you can get it out easily.