The reason may not be what you think.
Most of us have been there. You just want to make your partner happy and/or you just want to go to sleep. So you fake it. (If my husband reads this: sorry honey, but it's true). In a 2010 study, 67% of heterosexual women admitted to occasionally faking an orgasm. The rub? (no pun intended) Only 20% of men think their partner has ever faked it. But us women aren't the only ones faking it -- 28% of men report faking orgasm as well (although as a urologist I still can't quite fathom how a guy can fake it as easily as a woman, but I'll get back to you on that).
So why do us women fake orgasms? One reason might be basic biology: reproduction. One study showed that the more vocal a woman was helped speed up their partner's orgasm, which may increase the likelihood of conception. Another way to think about it is that faking an orgasm is a way to manipulate a male partner's behavior. (I mean, why wouldn't I fake it if I can get something out of it???) A less self-serving reason women might fake orgasm is simply not to hurt their partners' feelings, or even make a woman feel "normal." The inability to achieve climax may not only affect a partner's feelings, it can also make the woman feel abnormal as well. Although movies make it seem that female orgasm is inevitable every time she has sex, in reality 75-90% of women do not consistently orgasm with a partner, and 5-10% of women don't orgasm at all.
Cooper and colleagues actually created a Faking Orgasm Scale for women in an attempt to determine women's self-reported motives for faking orgasm. Their group found that women have both self-focused and relationship-oriented motives when faking orgasm. They describe four factors related to faking orgasm with intercourse: 1) altruistic deceit; faking out of concern for a partner's feelings, 2) fear and insecurity; avoiding negative emotions, 3) elevated arousal; a woman's attempt to improve her own arousal by faking orgasm, and 4) sexual adjournment; faking it to end intercourse.
Whatever reason a woman has for faking it, it's her reason. And that should be good enough for all parties involved.