Why does spironolactone cause gynecomastia?
Spironolactone is a relatively non-specific aldosterone antagonist, so it can also bind to aldosterone and progesterone receptors. Inhibition of these result in relative increase in estrogen influence on the breast tissue, causing gynecomastia.
Eplerenone is a more receptor-specific aldosterone antagonist, so does not cause gynecomastia as often. Other meds used for HF that also (rarely) cause gynecomastia include ACEIs and digoxin.