Say you’re awesome at making widgets. You’re pretty good to decent at a number of things, but you can’t build houses or garden or cook or sew, you have no interest in serving people food, you’re a terrible painter, and you hate shuffling papers in a mindless cubicle-job. Your real talent, though, is in widget-production; man, can you make a mean widget. So you decide to seize your comparative advantage in widgets to make and sell them for a living.
Well, you’ll need a workshop to make those widgets in and a storefront to sell them from, so you’ll buy or rent out space from someone who owns a building with the right accommodations.
You’ll then need equipment, which at some point you purchased from people who sold them. But now that you have the bigger shop, you buy even more.
You’ll need steel, screws, some gears, wood, cables, molded plastic, batteries, and other materials which you will use to make your widget. All those goods were previously sold by someone (and were made by someone else, whose component parts were culled or mined or created by yet another someone else).
You’ll need to hire people to help you make the widgets. Hire people to package them. Hire people to mop the floors at the end of the day.
You’ll need to buy power and communications and transportation capabilities from others still. Maybe hire a fumigator to tackle those termites, a plumber to repair a busted pipe, a chef to cater the grand opening, and a glazier to fix that broken window.
Of course to afford all this initial capital expenditure, you might need a loan from someone who saved the money first (that is, deferred their consumption) in order to lend it to you because they believe in your idea and your ability to implement it, and therefore your ability to pay interest (that is, the price of borrowed money).
And then you’ll need customers who value your widget more than the price they’d have to pay, or they wouldn’t freely make that trade.
And since you were the “customer” or exchange partner in all those previous transactions that led to that sale, you therefore preferred those goods and services more than whatever you gave up or you wouldn’t have made the trade.
Furthermore, every exchange you make is based on the fact that someone else was catering to your demands. In fact, you chose to make widgets because you’re better at that task than you are at making your own house or clothing or food - so with what you earn by fulfilling others’ demands, you can hire others who are good at what you are not. Our success, thus, comes from serving each other’s interests through assistance, inclusivity, tolerance, and cooperation.
Self-interest, as I’ve noted, is the motive behind all human action. We all observe and experience the world through the prism of our own minds and aim to achieve our own desires (for good or ill). Preferences are subjective and we all act in order to reach (or, more accurately, attempt to reach) some desired end. Any action that is voluntary reveals our preferences, which is to say our voluntary actions are always what suit us. As such, the state creates mechanisms with which individuals can pursue their self-interest at the expense of others and be inoculated from the consequences of their actions. The alternative is a society in which private property is respected and people face consequences for their mistakes and misdeeds (both from liability and competitive pressure); this is a society that rewards - and causes to flourish - peaceful cooperation and mutually beneficial exchange. In short, the free market best harnesses our individual, narrow self-interests to serve others.
So for any given market activity, there will likely be far, far more people you cooperate with than people you compete against. Free markets are the opposite of isolating. It is the state that monopolizes force and chooses aggression over persuasion. It is the state that puts limits to peaceful activity and raises barriers to consensual exchange. It is the state the forces us to compete with each other as to whose ideas will dominate each other’s lives instead of allowing us to make peaceful decisions over our own lives for ourselves. The state divides us, the market brings us together. Because that’s what the market is: the interconnected nexus of mutually beneficial exchanges among consenting individuals.
Free marketeers have been saying this for so long. It’s intellectually lazy to continue to assert that free markets/libertarianism promotes social isolation. My favorite writing on the topic is Leonard Read’s wonderful essay, “I, Pencil" wherein he writes how no single person can produce a pencil. The production of a pencil brings together people from around the world in pursuit of their own self-interest. It brings people together who might otherwise hate each other but instead put aside their differences in order to better themselves. This is why economists say things like "trade is made of win" because it literally creates wealth.
Seriously if your criticisms of free markets or libertarianism is because it “promotes selfishness or isolation” then it’s clear that you haven’t really put much thought into your opinions and are just spouting out what you’ve let other people think for you.