Bingo, you nailed it. (No double entendre intendred.) It's impossible to give yourself completely over to the moment and experience the highest peaks of sexual excitement when you're worried that your pleasure will cause you to disappoint your partner.
I worked once with a couple in therapy who were frustrated because the man suffered from PE (premature ejaculation), leaving his wife frustrated. I asked him, "Does your tongue lose its erection, too?" By the time of their next session, not only was she feeling satisfied, but his PE problem had remitted.