The absolutely most selfless thing you can do: become a mother.
Especially a stay at home one.
Here in the US - and other countries - people will look down on you for being lazy, for making the choice to be a stay at home mom. If you work they'll be more sympathetic, but HA, it's so much harder to find a job because "you might get pregnant", heaven forbid you actually have kids.
People will say you're selfish. That you've only had kids so they can take care of you later. Let's see. Let's go through an 'average' life (aka me pulling numbers out of my butt and using cliches and experiences I've known people to have. This doesn't happen to everyone but it's happened to a good number of moms):
You have kid. Baby lets you sleep, maximum, a couple of hours at a time. If you work, you know you ain't getting enough sleep. Even staying at home may be a struggle, what with cooking and cleaning - your spouse will expect it. After all, you're at home.
Toddlerhood. You can sleep the whole night through! But your kid is everywhere. Getting into everything. They're getting in trouble, not to mention if you have pets you have to be watching them 24/7 because pets and kids don't mix - you never know what may go wrong.
Your kid goes to elementary school. You clean when they're gone, but in the morning you have to make lunch, cook breakfast, get them dressed and on their way. And if your kid doesn't like school? Your job's ten times harder. When they get home, you have to wipe away tears if that day was hard, encourage them to be understanding of others, and provide hugs.
Middle school. That's hell for kids. I don't know anyone who's actually liked middle school; it's been awful for EVERYONE. And because your kids are going through hell, you have to go through hell to. It's taking care of tears every day. It's homework help. It's frustration and calling teachers and having to sometimes literally drag your child out of bed in the morning.
High school. Half of everyone likes it; the other half despises it. You still have to make sure they're all ready, even if you don't have to dress them and make lunch and all that. You still have to listen if they're upset. Oh, yeah. And you're going to be called the worst of names, cuss words and stuff they come up with their own. Teenagers are extraordinarily clever at insults. Plus "I hate you!" and your child will complain about being tired and hungry all the time and if you do anything but accept this and offer condolences your child will be pissed.
College! You think you're free. Ohhh, no. Your child may be away or living at home with you, but either way you still have to take care of them. If they're at home, you'll have to help them get ready for their classes by waking them up. If they're away, you have to call them every day. Sure, they may be over that teenaged angst, but expect hours of them complaining about something. Expect "I miss you"s. And during holidays they're going to come back and eat everything. They were used to ten cent ramen and sometimes burnt toast; they will love the food you provide for them and compliment your cooking, constantly, but they will eat you out of house and home if given the chance to.
FINALLY they're out on their own, living life. They still give you daily calls. And marriage? HA! They'll be asking you for constant advice, the best ways to ask to get engaged, how to solve a fight. "Mom, I need help with x" is your constant call to action.
Then grandkids. You'll have to babysit them, and if you don't take care of them the same way your child does they'll lecture you and glare and be irritable. Plus, they're still coming to you for advice.
Let's say you live to be 100, and around 80 you start to go a little senile. The grandkids are growing up and getting kids of their own. Your children may just pay to put you in a nursing home, and be upset that they have to use their own money to take care of you. They may have you be with them and take care of you well. That's still 20 years of care that's considerably easier than the 80 years you put into taking care of them. Yes, it can get difficult - but even when they're supposed to be taking care of you they'll still come to you for advice, still talk to you.
And judging by my mother, even when you're dead they'll still want to talk to you.
Motherhood's selfish? It's easy? HA! It's a full-time job, twelve AM to twelve AM, weekends and no vacations. You have it for 18 years, maybe up to 25 years, of being super-difficult and then it gets easier --- but you're still working. You grow to expect middle-of-the-night calls from drunken college students or the late-night fights with their spouse. You don't get to quit. Ever. And your whole life you'll be doing this job, not to mention the possibility of other jobs if you work elsewhere. Sure, it gets easier, but that doesn't mean it's not a lot of work.
Motherhood is hard and absolutely the most selfless thing you can do. Quite frankly, I don't blame ANYONE for choosing to never have kids. I do, however, get pissed off if someone's going to call motherhood 'easy'.
This? This is the easiest motherhood gets, that right above. Single moms? Mothers who have children who have mental and/or physical issues? HAHAHA GOOD LUCK.
...and I still want to be a mom. Go figure *shrug*