I've had sex that I was really enjoying to the point where I wanted to cum, only to have my partner sense that I was getting close and stop what he was doing. I understand that his intent is to slow things down and make it last longer, but when I get to that point, I don't necessarily want to slow things down, I want to CUM. If the feeling of impending orgasm fades, I'm unlikely to get it back again, so them I'm stuck continuing with sex that I've lost interest in, trying to get him to cum so that I can be done with it. Then after he cums, he wants me to cum, but by then I'm like, Sorry, buddy, ain't gonna happen; the opportunity for that came and went a half-hour ago, but it's too late now.
Because of this, I only reach orgasm in slighly more than half of my sexual encounters.