Because the prostate is right next to the urethra just below the bladder. So when it gets bigger it cuts it off or make it smaller. See related link.
The enlargement of the prostate gland interferes with a male's reproductive function in the sense that it makes it impossible for the gland to produce semen.
prostate.
prostate enlargement
An enlargement of the prostate gland may restrict the flow of urine from the bladder. The prostate gland grows larger due to an increase in the number of cells, hyperplasia.
The role of the prostate is to produce a liquid that mixes with semen and helps the sperm move through the semen into the woman's body. When a prostate becomes enlarged, it can effect the production of the fluid. In most cases of BPH, as the prostate becomes enlarged, it impacts your ability to urinate, due to putting pressure on the bladder. Also, an enlarged prostate can lead to prostate cancer if not treated.
Prostatic Hyperplasia
Benign prostatic hypertrophy
As the prostate gland becomes enlarged, it presses against the bladder and constricts the urethra, which is the tube that runs from the bladder through the prostate and out through the penis to remove urine from the body. As the bladder and urethra are constricted, it interferes with the easy flow of urine from the body. Not only does protate enlargement constrict the urethra, it often leads to infection of the prostate and in some serious cases operation is necessary.
inflammation of the prostate gland in men (prostatitis) enlargement of the prostate gland (benign prostatic hypertrophy) bladder or pelvic tumors
The urethra passes through the prostate, and when the prostate enlarges it compresses the urethra. This makes it hard for the man to start urinating, and then when he starts he doesn't get it all out, leading to a sense that he is always full (urinary retention).
No, a benign prostatic hyperplasia (BPH) does not cause a cancerous enlargement of the prostate. A BPH is not cancerous. It is defined as a noncancerous enlargement of the prostate gland.
Over time, the prostate, a donut-shaped gland below the bladder, enlarges. When this happens, it may interfere with the passage of urine from the bladder out of the body.