I don't mean to be critical, but your question doesn't have a lot of details, so it's a little hard to answer. For example, at what age did you start masturbating? How often do you do it - a few times a week, several times a day, or what? It's possible you just got bored with the same routine, especially if you're doing it too often. Try 'backing off' a little - that is, try to go a few days without any stimulation, then when you do, your urge will have built up to a high level & you'll enjoy it a lot more. I know at 16, many guys are masturbating at least daily & often more than once a day. Anything can become unexciting if you always do it the same way & too often. If you can provide a little more detail in your question - technique you use, frequency, how long each 'session' normally lasts, those things might be clues to where the problem lies...