Well that's why you change it up! Masturbation can dull itself if you don't find new ways to rub, jack or just get the friction going
+ Usually the feeling of wanting to hurry the ejaculation is the leading cause for male premature ejaculation
, which is the same thing as climaxing too early. This is a psycological thing and you have to train yourself to slow down and enjoy it.
For men (not sure for women), if they keep hurrying things up, it might lead to type 2 premature ejaculation*.
*Type 2 premature ejaculation isn't worse, it's basically condition that has developed, not one that has been there ever since puberty (type 1)