After my vasectomy at age of 33 i had a very increased sexua| activity. I think it was surely psychological: the snip was the sign of my virility and there was no fear to pregnancy anymore, I felt more attractive. At the other side I think there are some medical reasons too: since the sperm can't escape anymore the testicles and epididymis are a little swollen and more sensitive, the shaved sack too.
But there can be a more important reason: in the '20 there was a surgeon called Steinach who perfomed unilateral vasectomies to ... rejuvenate old men. He cut off one of the cords so the man kept his fertility but the sealed testicle began to produce more testosteron and the erections became better. You can find it on the web by 'Steinach rejuvenation"...