It's the classic "Socialism lacks incentives" argument, of which there are two points to be made, the first being that Socialism is not Communism.

In Communism, it's "to each according to their need." In Socialism, it's "to each according to their contribution", which isn't unlike Capitalism in that you perform a task and in turn receive; in fact, this method is step up from Capitalism, because, for once, hard work will actually pay off.

Secondly, this argument presumes that people are only incentivized by material gain, when there is much to be said about forming friendships, finding love, raising children, gaining admiration and glory, improving your talents and skills, and generally finding one's place within society. Purpose and meaning we desire just as much as food and water.

Now, this argument naturally tends to devolve into "Well, who is going to pick up the garbage and maintain the sewers?" I've always found this argument to be amusing, because you'd think in a Capitalist society these sanitation workers would be the wealthiest laborers around, but in truth they are often paid poorly, if not modestly. But regardless, one would be surprised by what acts people perform for little to no compensation; I can personally attest, as my friends and I volunteer to clean up trash at our local national park. What do we get out of it? Nothing but the satisfaction of exercise and cleaning up the environment. Surely there are people out there who would be glad to pick up garbage or keep the sewers running.

If one still remains skeptical that these people exist, then I offer two more responses: First, in an ideal Communist society, technology will have progressed to such a point where unsavorable tasks are automated and performed by machines. This almost seems inevitable given enough time. Secondly, we can take a hardline Stalinist stance: everyone produces trash and waste, therefore everyone is obligated to labor in the continued maintenance of sanitation infrastructure. Refusal to do so will be considered a crime. This is not unlike the involuntary military service we see in Capitalist countries, except