Hormones like dopamine, oxytocin, and prolactin can change after a man ejaculates, which can cause mood changes. When you get sexually aroused and ejaculate, your dopamine levels rise, which makes you feel good. But dopamine levels drop after ejaculation, which can make you feel less relaxed or upbeat for a short time.
Oxytocin is also released during ejaculation, which helps people bond and feel close, but its levels may drop later, which can affect mood. After ejaculation, prolactin levels rise, which can make you feel good but may also cause a refractory time during which you can't do anything sexual. These changes in hormones, along with things like tiredness or mental states, can affect a man's mood after ejaculation. Moods can range from happiness and relaxation to short-lived sadness or boredom.