Sex is very pleasurable. Sex allows one to condition themselves, like a habit or addiction. This is why alternate forms of sex ranging from internet porn to barn yard critters and all other things, once in the closet, can be highly addictive, to where it almost appears to be a genetic based instinct like procreation. The drug addict, using a new designer drug that the body cannot possibly have evolved genes for, can nevertheless appear to be driven by an instinct, due to a pleasure conditioning. This conditioning can motivate them as strong as lust.
Love is different, since love is not always pleasurable, therefore it does it always result in addiction reaffirming feedback. If you loved a difficult child or a cheating spouse, this is not the same as pleasure all the time. It is more like a blend of pleasure and pain, which has the affect of moderating compulsive subroutines. This allows for more will power and choice. This is why love can extrapolate further. The person who does some drugs, but can take it or leave it, will not form the same compulsions.
Sex and lust is designed to help create an addiction to another person. But like all addictions, these will have a finite life expectancy of optimization, until the slave becomes the master; ease is gone. The goal of this is to break the subroutine of addiction, with the hope that love and self control appear. In the old days, there was saying that sex does not last, but cooking does. The road of lust can lead to nurturing and love.
