If you take time to investigate someone who appears to be "naturally" funny, you will find that they developed their ability to be funny through exposure and experience. Either through watching hours and hours of television or exposure to friends or family who also had exposure to comedy. Through many of my own observations with multiple students, I've found that we can recreate that kind of exposure and help them develop a spontaneous sense of humor that may not have been there before. This is accomplished through enabling them to understand the stimuli that makes people laugh, then giving them repeated exercises to help them automatize this understanding until it becomes "natural."