Among anti-capitalists, there is a common accusation: that the cornerstone of capitalism is "keeping the poor in their place" while making the rich get richer. This is, to be frank, patently false; while there are probably a few (evil) people who want to keep people poor, the vast majority certainly wouldn't mind other people getting rich, and most really do want to see people's lives improve, though not at the cost of their own.
In a capitalist system, this is possible, because wealth is not considered a finite resource like physical objects are. While it is certainly traded, it is also created by the workers, and so while there is a finite amount at any given time, that amount is always growing. This is due to money bring treated as an abstract concept, not tied to the physical realm. It's also considered to be the main advantage of capitalism: because there is growth, there is always improvement, as opposed to a socialist system which lacks incentive to grow and therefore necessarily stagnates.
However, this can be broken. The reason is that the driving force of capitalism is competition. This is what induces companies to both improve their products and keep their prices low: being able to stay viable in the marketplace. As a side effect, though, it makes wealth more accessible to more people. Also, as improvements are made, barriers to entry are lowered; this is what makes entrepreneurship possible.
However, if that competition actually ends, the incentives are all lost. Under monopolistic conditions, there is no incentive to improve, nor to keep prices low. In fact, in a monopoly, the incentive is towards not doing these things. And so, it doesn't happen. When that occurs, barriers to entry rise, as do prices, hurting the people and denying them the opportunities they deserve.
This, in the end, is the real problem with laissez-faire capitalism: monopolies can and do occur under such a system. In order to keep things running smoothly, therefore, at least some regulation is a must, to ensure that competition, and with it the impetus to improve, stays a constant. Not necessarily enough to be called socialist (indeed, great care must be taken to avoid economic stagnation), but a balance must be struck somewhere.