When I bench press, my shoulders hurt like hell. One of the regrettable side effects of years and years of weight training is the build-up of adhesions in soft tissues and structures.
Adhesions are a result of the load used and the total volume of repetitions.
In other words, the more sets and reps you perform and the stronger you've become, the more adhesions you've developed. These connective tissue buildups can take place within the muscle, between muscle groups, or between the nerve and the muscle. Adhesions can occur in any muscle structure but the one most often responsible for bench-press induced shoulder pain is the subscapularis muscle. You don't need to become the Grand Master of Yoga, though. Regular P.N.F. stretching of the shoulder girdle before your upper body workouts will do wonders for keeping your shoulders healthy and functional.