According to The Register, levels of the “satisfaction” hormone prolactin do not reach as high a peak following manual stimulation in men as they do after purportedly procreational activity with a partner.
The original research published in Biological Psychology could explain what The Register describes subtly as a “niggling dissatisfaction” following the former approach to gratification. Needless to say, the remainder of their article is anything but subtle and the link above should only be followed if you’re feeling up to blatant sexual innuendo.