Disclaimer: I am no expert in Marxist thought. However, seeing the massive amount of suffering and death caused by regimes based upon its premises and the fact that many people today espouse these principles, I thought it worthwhile to try and understand it.
Businesses are just groups of people
If I'm not mistaken, much of Marx's economics is based upon "surplus value", "ownership of the means of production by the bourgeoisie" and other well-known terms. It does seem, at first reading it, that these dynamics do lead to the proletariat (the lower class which does not own the means of production) never being able to ascend to the "higher" class.
Well, left at that, it does seem rather ominous and it's no surprise really that many young people (including myself) at some time in their lives take this as a perfect explanation of everything that's bad in the world.
Now, ok. Well, for starters, this is not what we find in those countries which have most intensively embraced liberal economic principles (market economies), such as the U.S., United Kingdom, Australia and Hong Kong. Actually, those are some of the countries with the highest quality of life and GDP per capita. While there is some variation of income in the population, even the very poorest individuals have a better standard of living than the majority of socialist/communist states today or in the past, not to mention individual freedom.
Now how did this happen? Marx's argument seemed to have some consistency after all.
Well, what happened is that people (and groups of people, by extension) are naturally greedy. Their prime motivator is personal gain (financial or otherwise).
In being greedy, each company/industry or capitalist individual, will try to maximize their own income. What happens in reality is that companies will engage in competition over who gets the better employees. The most obvious consequence of this is that wages are pushed up:
greedy capitalist A wants to maximize his profits and knows that he needs efficient and hard-working employees to do that. Now, in a free market, there are many other companies and heartless exploiters around so company B also wants to maximize its own earnings. So they both want bright young employee X to work for them. They will each, in turn, offer as much money as they can to get employee X working for them, which is good for employee X as he will have a nice salary.
Not because companies desire the "well-being" of their employees, mind you. It all started with individual interest.
Now the other side of this story is that employee X is only motivated by personal gain as well (he loves his employer no more than his employer loves him). So he might actually try to work harder and more efficiently if only to make himself more attractive to his employer, which might mean higher wages for him. If his current employer does not raise his salary, there may be other companies/greedy capitalists willing to pay more to have him on board.