In much the same way, because of social stigma, many men may feel embarrassed to admit they like to be dominated in their fantasies. When in fact this is also very common. Men are often expected to be the dominant ones, and being submissive goes against the grain. This is why it is not necessary to disclose specific details. But knowing the fantasy and admitting it to yourself can open up a door towards understanding your drives.
Understanding where our fantasies come from is not necessary unless you want to know more about what it is that actually is driving you. Why are some people more dominant and others more submissive? Why do some like to watch, where as others like to perform, or be watched? I believe it has to do with our upbringing and conditioning in childhood. During our formative years, about ages 5-12 we internalize a lot of messages from our primary caregiver. If our primary caregiver is over protective, for example, we may develop a fetish to be more of the submissive, even humiliated type. Sometimes a specific event during puberty may also condition us towards certain stimuli.
All in all, it seems to me that our fantasies represent something in us that we either wish to work out and gain mastery over, or something that balances out our lives. Therefore, our fantasies are not to be ashamed of, but rather to be appreciated. Thus, our fantasies help us. Once you recognize this and allow yourself to admit to yourself your true desires, can you then start to slowly let your partner in on how he/she can contribute to making your sex lives more fun and fulfilling.