Society itself functions on the exchange of money. As such, the primary motive for people to work hard at their jobs is, in fact, to earn money.
Humans have five basic needs: food, water, clothing, sleep, and shelter. Ignoring sleep to an extent, the remaining four cost money. Whether it be buying groceries or buying ready-made food from restaurants, the transactions require money to be fulfilled. The buying party must pay the selling party in order for the transaction to be fulfilled, and in most cases in modern society, the payment is done using money. This applies to other needs, as well, from buying clothes to having a roof over your head for shelter to paying the bills and the rent associated with the shelter.
Therefore, human beings need money. It is an absolute necessity. Where, then, does that money come from? For most people, they earn their living from their occupations. The employee completes the necessary tasks, in accordance to their contract with their employer, and the employer compensates them for their work. However, if the employer finds that the employee has not been fulfilling the necessary work, or has not been completing the work to a satisfactory level, they have the right to deduct the wages of their employee. Conversely, if the employee goes above and beyond what is asked of them in their contract, there is a possibility that they may receive bonuses, which they may afterwards put into their savings. As such, people have a clear motive for working hard.
Aside from their occupations, there is also another method to earn money: to work for yourself. To elucidate, it is to create your own company and sell your product en-masse to buyers. This, too, can be viewed as a “job” to some extent, and requires a lot of work put into it for success.
Therefore, after taking into consideration the above information, it can be logically concluded that human beings require money to live proper lives, and the most straightforward method of earning money is to work hard at their job.