Can you please explain why excessive masturbation is bad?

Warning sign: It's only a problem if you recognize that you're obsessed with erotica and your sensual side to the extent that it's interfering with doing the things you must do to become / remain an effective adult. You won't injure yourself, but if this unwanted compulsion describes you, get with your physician -- a bit of rx, insight therapy, reprogramming & maybe referral will help out.

Excessive?: I wouldn't put a value judgment ("bad") on it, but how much time are you spending masturbating daily or weekly? Is this to porn? Are you able to relate intimately or sexually with live partners? Is your masturbation interfering with your work, other hobbies, family, taking care of your home, eating healthily, exercising, spiritual practice, etc? If so, it could be an addiction, and needs help.