I'm talking about non-lucid control of dreams, like "I want to dream about unicorns." No matter how hard I try I cannot get myself to dream about what I want to, or anything related to my day.

For example, I was having a lazy day and watched 3 vampire flicks. That night, I thought about vampires and wanted to have a vampire dream. Didn't.

How come some people can do this and others can't? Is there a way to improve my chances more?