One way to understand the wage curve is as follows. The labour supply of each individual is positively correlated to wages, therefore the higher is the hourly wage offered, the more hours an individual is willing to work. However, there is a limit to which every person would be willing to sacrifice an hour of leisure or rest, for an hour's worth of wages. Let's say that X is the maximum number of hours a person can work, and $A is the minimum hourly wage rate he expects in return. Any wage $B, greater than $A, will increase the worker's daily wage without increasing the hours of work. So if you need more hours of work than X, you need to hire more people.