i dont really think its nature. nature is to defend yourself - survival of the fittest. so nature would make us do self care in overdrive.
i think the biggest part is that when were young (typically) we see our moms do all the work. a lot of moms now a days work a full day then come home and do everything around the house and pull the 2nd shift. honestly, i dont think i ever saw my mom do anything for herself.
i think if we saw our moms do things like go to the spa, take a personal vacation, have the husband do some chorse while we rest - it would be a lot easier. but the truth of the matter is that, until recently, women have done about 80% of the work. so that teaches us at a very young age that we have to be superwoman. we have to work a job, plus take care of the kids, plus cook and clean etc...
now its getting better and a lot of men are helping around the house and with kids but thats with a generation who has already had the typical housewife thing engrained into us. i consider myself very feminist and i still find myself doing it. i still feel like if i want to buy myself a purse once a year that im being selfish. or if, god forbid, i want a pair of jeans... thats just too selfish. but i constantly have this inate want to make food for my boyfriend and give him back massages etc...
nuturing and self-destruction are 2 different things. we all have our own opinions on it. some women will always feel like they are supposed to be in the kitchen. now its our job to change that stereotype
my boyfriend constantly asks me to sit down and let him make me food, let him get me something to drink etc... and i have a problem with it for some reason. so even when i have guy who is trying to pamper me i wont let him.