The most important factor that determines whether a prostate cancer is "aggressive" or not is the degree to which the prostate cancer cells are abnormal. Extremely abnormal prostate cancer cells behave much more aggressively than near-normal cells.
The prostate cancer grade, often quantified using the Gleason scoring system, is a measure of just how abnormal the prostate cancer cells are. The more abnormal the cells, the higher the grade, and, generally, the more aggressive the cancer.
Cancers can also be more aggressive in men who have other health problems or weakened immune systems. In a sense, the body is not strong enough to fully block the growth of the aggressive cancer.
Allsbrook Jr WC, Mangold KA, Yang X, et al. The Gleason grading system: an overview. J Urologic Path 10:141-157, 1999.
Gleason DF. Histology grading of prostate cancer: a perspective. Hum Path 23:273-279, 1992.