Dopamine is not about the happiness of reward. It’s about the happiness of the pursuit of reward that has a decent chance of occurring.
Due to habituation, the value of a reward is always relative. It is relative to previous rewards, alternatives to the reward, and uncertainty of receiving the reward.
Anticipation is where the most dopamine is released, and therefore where most happiness occurs. This may be why the act of trying to get laid is often more enjoyable than the sex itself (depending on the partner).
Normally, dopamine is released during the signal that reward may happen, very little or no dopamine is released during the work phase (in order to get the reward), UNLESS the certainty of the reward is low. 50% seems to be the magic number since that is the most uncertain value!