There might be more than one reason, depending upon what is going on (or not) at the time.

If you are referencing this problem with masturbation then know that--
* distractions from outside can disturb your focus
* fear of discovery can disturb your focus
* changing the pace of how you are stimulating yourself will cause your level of arousal to pause or increase
* changing where you are stimulating yourself...
* incorporating a fantasy or not, or, changing the script...
* waiting too long to add lube if required, or, keeping a particular spot lubricated...

If you are referencing this problem to how you and your partner make love, then in addition to any of the above are--
* that you focus on the other instead of concentrating on the sensations you are receiving
* that your partner changes the type of stimulation or location of stimulation
* that your partner does not stimulate you in a way that you immediately require
+ that said, having a lack of communication or feedback (verbal and/or non-verbal) in order to convey what you need now/next

I hope this is of help. Got questions?

Welcome to the SI101 Board and its Forums. I hope you will enjoy participating. Please begin by familiarizing yourself with the FAQs, the Posting Guidelines section, and especially the Index, all found at the top of the main screen. The Index contains links to helpful informative insightful, as well as how-to articles that discuss the most common questions and concerns people ask about. In addition, if you click on the site's Home Page you will find even more information.

I recommend that you read the several articles that discuss the female "O" in all of its ramifications.

-doc