I think the main driver for one wanting to stretch one's anus to its limits is masochism. My pleasure comes from submitting to the ravager of my orifice, and from being desired in return. I don't think it's pleasure from direct stimulation, because if it's too large, it can be painful instead. And competition does not drive me, because I don't compare my "anal capacity" to others.
In a sexual situation, desiring--or at least tolerating--the penetration and stretching of one's orifices is only natural, because....how else is mother nature going to get two individuals to engage in the penetrative sexual intercourse so necessary for the survival of the species?
I love it when my orifices are ravaged and stretched by my loving partner, because I know that it is an affirmation of his animal desire for me, and this more than makes up for the soreness, aches, and sometimes bleeding after the "violent" sexual act. Reason says I should stop doing this because it can't be good for my health, but instinct usually wins.
I think the desire to be anally stretched, and broader masochistic drives, are a legacy of our evolutionary history, when sexual intercourse likely involved violence of males against females. Think bands of cavemen or hunter-gatherer males raiding each other and stealing females for sex, like chimpanzees do. The submission of one's body to violent or painful penetration and feeling sexual pleasure from it may have been advantageous for reproductive success in our ancient past. But hey, it could be worse, like the traumatic insemination of female bed bugs, or the sexual cannibalism of praying mantises and spiders.
It's such a primal instinct that sexual masochistic drives are present in both men and women today.