Aristotle said, "Man is a social creature." Humans have always lived in societies, be it small hunter gatherer bands, fabulous Aztec cities or sprawling concrete jungles like NYC. But what makes humans different from animals? Is it because we can communicate using complex languages? Is it because we write music, create art and write literature? Is that what makes humans funadmentally different from animals?
None of these are the fundamental reason that humans are different from animals, though they are symptoms of this fundamental difference. What makes humans fundamentally different from animals is how humans meet their needs.
Engels said (paraphrase) "Man must first eat, drink and find shelter before he can worry about art, religion and politics." This is why Marx and Engels were materialists, meaning they looked to the material world and its economic base to explain the world of ideas and its political, religious and social superstructure.
Since "Man must first eat, drink and find shelter before" anything else, examining the way people obtain a livelihood is essential. More specifically, since "Man is a social creature", looking at how things are produced in society as a whole is of the utmost importance to us.
Collective labor is the reason why language developed. How are you supposed to work and get anything done if no one can communicate with one another? In the Bible, God sabotaged man's dream to build a tower reaching into heaven by making all those involved speak different languages, rendering their collective labor power useless.
Human labor power and the creativity and inventiveness that come with it lie at the heart of human nature. Human labor, given freely, most closely resembles the labor put into a hobby (unless you are one of the lucky few to enjoy their work).
We have already established that labor power, the ability to work, is exploited under capitalism. Labor power is sold by the worker, the proleterian, to someone else because he has no other means to survive. This labor power is sold to the capitalist, the bourgeois, who purchases and by the very act of purchasing it, exploits it.
Labor power is bought, sold and exploited. Labor is alienated from the laborer. The laborer has lost control over his own labor, which he is compelled to sell and so it becomes something that is alien to him. He does not give his labor or perform his task because he wants to, or because he enjoys it or because he knows it would be useful, he performs his task because he has to, or face starvation and ruin. He has no control over whether he wants to work or not, what conditions he works under, how fast the assembly line moves or what is produced. Labor, the expenditure of labor power, is not given freely - it is extracted.
Engels, in a passage of his book the Condition of the English Working Class explains what is sometimes called "wage-slavery" thus:
The only difference as compared with the old, outspoken slavery
is this, that the worker of today seems to he free because he is
not sold once for all, but piecemeal by the day, the week, the
year, and because no one owner sells him to another, but he is
forced to sell himself in this way instead, being the slave of
no particular person, but of the whole property-holding class.
This is why work sucks. Labor that is exploited or usurped is no longer a source of enjoyment. It is de-personalized and turned into boring, soul-grinding drudgery. Work sucks because it alienates the worker from the products and the process of production.