Well, I'm about to prove that you do it too. Behave unfairly and selfishly, that is. And there's nothing wrong with it.
Look at your feet. Have you got shoes on? If not, then look at your drawer or wherever you keep your shoes.
Did you buy them because you thought they had a good benefit-cost ratio, because you liked the way they look, because it's what's trendy these days, because they were comfortable (i.e. selfish reasons) or because you thought it was the fairest thing to do?
Well, if you think people should act fairly and not put their own interests first, then you should probably have bought the ugliest, less durable, less comfortable and priciest shoes in the whole shop.
That's because those shoes might have been made by poor people who just aren't good at making shoes. Poor them. Maybe they hail from a poor, faraway country and the cost to ship it here has made the price twice what the others are. It would be fair to buy those shoes because then people who really need the money would get it, no? Now that would probably be what's worst for you, for that would make you spend more money and have a sore foot and whatnot. But then you would really be behaving very fairly and altruistically.
Better yet, if you really wanted to help people in need, you would probably want to go 100% barefoot and donate all the money away to an NGO of sorts. Now that would be altruistic. Be willing to deny your needs like that just for the benefit of others.
But of course you won't do it. You will just do whatever it is that's best for you in your private sphere and when nobody's looking but, when you are in public and/or with your leftist friends you'll say that it's so unfair to dismiss people because they aren't making profit for a company; or that it's unfair there exists a big income inequality in most countries. Or that it's not fair that some people live comfortably while others live in a slum.
When you catch yourself saying that, remember this and think whether you're really willing to do what it takes to be altruistic (not just to talk the talk but also walk the walk) and, when you conclude that it's not humanly possible to do it other than turning into a non-person and living your life for the others entirely, you might live better with your selfishness, and maybe even come to value it.
Guess what: it's not wrong to be like that. It's not wrong to be selfish. It's the natural thing to do. It's what all other species in nature do. It's just how we, humans, are. Why can't we be honest about it? Nature has made us like that. The universe has made us like that. Why fight it rather than accept and cherish it?