Nowadays, almost everybody is forced to sell their labor power in order to survive (or, at least, to avoid hardship and official harassment). This necessity seems so natural to us that we seldom think about why it exists and how it came about.
Yet, historically the situation that (almost) everybody has to sell their labor power is rather new—it only emerged with capitalism. Wage labor and capitalism depend on each other: Without capitalist companies, there would be nobody (by and large) to sell your labor power to, so wage labor could only be an occasional phenomenon (as it was before capitalism). And without wage workers, capital accumulation would be impossible, since their surplus labor is the source of profit (and of its derived forms, interest and rent).
That wage workers are forced to sell their labor power (due to a lack of alternatives) is just one side of the coin. The other side is that they are allowed to do so—in contrast to slaves, who cannot sell their labor power since they themselves have already been sold by others.