Why does the prostate get larger in bph?

Hormones. Prostate enlargement is a normal process of aging in men. It is under the control of hormones (testosterone, dht) and genetics. The degree of enlargement is variable from person to person and the symptoms it produces is highly variable from person to person, even if they have the same size prostate gland.
Enlarged Prostate. The prostate is a gland that will grows throughout the life of the male. It grows in response to testosterone indirectly through DHT.
AgingGeneticsHormone. Prostate enlargement in BPH is due to aging, Geneics (as it runs in families)and Hormonal influence by Androgens.
It is... BPH (benign prostatic hyperplasia) is a tissue DX (under microscope), notably an increase in cellular elements of glands, appearing in >90% of men >85 yrs; it's related with high dihydrotestosterone (dht), leading to more cellular growth ; increase in smooth muscle tone, so resulting in poor smooth muscle relaxation and more tissue bulky obstruction. So, bladder has to work harder to squ.