There's plenty of reason to fake an orgasm, the most prominent being the expectation for there to be one.
For the longest time, I couldn't cum and it disappointed countless lovers, and on one occasion, I was dumped for it. It "let him down" and he wanted someone who could show him he was doing a good job, which case in point, meant orgasming. Plus, there's such a social stigma of, "Oh that guy can't make his girl cum, he's shit in bed." There's a ton of reason as to why an orgasm is expected to happen from both parties, a lot of it being society putting such a pressure on it being the grand finale and the reason to be intimate at all.
It's true, orgasming feels awesome. But when I couldn't cum, I never missed it because I'd never felt it before. I still enjoyed sex, copious amounts of it, so I always thought "What does it matter? Why is this so important, shouldn't it bother me most?" If your partner thinks an orgasm happening or not happening is a status report on the way they're performing in bed and you can't seem to change their mind, faking it is usually the route to go when you just want to put them at ease and assure them it isn't their lack of great giving skills that's the problem.
I haven't had to fake orgasms much since I met my partner now, who knows my body a little bit more and understands that I need specific things in specific places, no matter how un-romantic it can be, to orgasm and that she has little to do with it happening at all.
Sometimes it's how relaxed you are, or your amount of stress, or you're too relaxed and can't tense enough for it to happen. But the most important thing to know is that it's just your body's choice or how it's feeling that day.