anonymous said it perfectly (hormones, etc.)... Think of it as like climbing a tall mountain - on the way up (arousal) you're going higher and higher with sexual tension
(requiring lots of energy, too). Then you cross over the peak (orgasm) and head down a very steep slope on the other side, where your body is programmed to rest and recover.
Thank God for these wonderful mountains - try to enjoy all parts of the trip, up - over the peak - and back down!