It's lack of excercise.  Yup .that's it . Diet is important of course...But if you just sit on your ass all day at home expect your ding dong not working as effectively.  Start even with 10 minutes walk.  Best option of course if you join a gym . But you should do some cardio as well . You know why?   Because What makes an erection?  It's the blood circulation. If you exercise you improve your blood circulation.   So no excuses..especially if you are in an intimate relationship with a girl ..this can be embarrassing..just move your ass to the nearest gym in your city .  Any questions welcomed .thanks!