Consumerism causes a kind of alienation, in the Marxist sense: When workers are separated from the products of their labor and compensated by an hourly wage, they can't find satisfaction in their jobs or the remainder of family life. Thus they turn to acquiring capital as the only form of self-fulfillment.
We work only to accumulate stuff and in turn the accumulated stuff dominates us, further distancing us from non-commodified things like relationships, joy, and community. Labor "is therefore not the satsifaction of a need, but only a means for satisfying needs external to it," Karl Marx wrote in 1844.
"The less you are the less you express your own life, the more you have," Marx argued, "the greater is your alienated life, the greater is the store of your estranged being," Stuff is therefore the enemy of happiness, and not just because it's crowing your apartment, but because it's part of this larger alienating system.