At the core of libertarianism lies idea of "screw you I've got mine". Behind all libertarian talk about "non aggression principles" hides desire to disavow any responsibility for how one's actions impact other people.
Take externalities as an example. Factory operating own boilers releases dust particulates into air, which have detrimental effect to health of everyone else, not just factory owner (assumption: owner actually lives anywhere near). Externalities are a market failure, that need to be corrected by government, in case of dust particulates by enforcing environmental regulations. Within framework of economic theory, correcting market failures is seen as adjusting prices to real cost: when part of the cost of manufacturing is being borne by someone other than factory owner, overproduction arises (because production costs do not reflect actual costs), which can be corrected by taxation or regulations. From social viewpoint, behaviour causing such externalities can be seen as incurring huge social costs and infringing on basic rights of everyone else. Socialists, leftists or collectivists will argue that this needs to be regulated. Economic liberals meanwhile usually claim that such regulation infringes on economic freedom of factory owner, completely disavowing fact that factory owner infringes on much more basic rights of everyone else (life, health, freedom from oppression: depending on size of nearby population, it could easily be that life-years lost due to health loss would be equivalent to life-years lost due to murder).
On the flip side, positive externalities also exist, linked article gives example of beekeeper keeping (obviously) bees, which through assistance in pollination increase yield of nearby crops. Again, economic liberals argue that nearby farmers who benefit from the bees owe nothing to the beekeeper and at best, can offer to pay him if they feel like it. Through opposite framework, correct solution would be to subsidize beekeeping, making beekeeping more economically attractive, thus increasing frequency of beekeeping through use of those market mechanisms that do work.
Lack of minimum wage, through abuse of balance of powers between owners of means of production and workers, often leads to wages below survival threshold, especially in low expertise occupations. Wages so low, make workers in such field even more susceptible to private oppression: threat of losing their already barely paying job only increases power imbalance and leads to many types of abuse including psychological, physical and, obviously, economic. Libertarians disavow responsibility for any of that, and in fact, consider leveraging imbalance for the purpose of economic abuse as right.
Therefore, since Libertarianism is a stance which declines responsibility for how one's actions impact other people or society at large, the opposite is stance which emphasizes (and enforces) responsibility for state of society and well-being of other people.
As a further note, I'd like to point out that, despite possible stereotypical connotations, collectivism is not limited to the left side. Left-collectivism will usually take form similar to class consciousness (goal of helping/liberating all workers, no matter their race, nationality or ethnicity) or similar, while right-collectivism usually takes form of nationalism.