As men age, their prostate naturally grows to some degree. This, coupled with a diet that is high in animal fat, such as meat and dairy products is thought to be a factor in developing BPH. Ethnicity is another factor that is thought to play a part.
As the prostate grows, it pushes inwards and restricts the natural channel that exists through the prostate, connecting the bladder to the urethra. With the channel restricted the flow of urine out of the bladder slows and the bladder will not completely empty.