Jiang M. Periodic changes in serum testosterone levels after ejaculation in men. Sheng Li Xue Bao 2002 Dec 25;54(6):535-8
Apparently if you don't ejaculate for 7 days, there's a spike in serum testosterone. It will reach 150% of the normal levels. But only once, and only after 7 days.
Think of it as an evolutionary kick in the pants. "What?!? You haven't been screwing? GET OUT THERE AND FIND SOME VAGINA! Just for good measure we'll make you hornier and more aggressive for a bit."