People think that desire motivates growth and development of mankind, a point that is not rejected by Buddhism either. It is said in the Mahayana scriptures that there must be self-grasping as great as Mount Sumeru in order to attain Buddhahood. Normally, self-grasping is condemned in Buddhism because it can cause greed, anger and delusion to arise. But here, it becomes the impetus to seek liberation; to certain extent, this is also a kind of desire. When desire to attain Buddhahood is absent, aspiration to learn Buddhadharma, to achieve enlightenment and to benefit sentient beings will not happen either.
Genuine Dharma practice is a big, demanding and time consuming project that needs sufficient courage and drive to achieve its goal, and the source of this courage is the desire to attain Buddhahood. Arhats, lacking such desire, do not have the motivation to reach Buddhahood and hence seek only liberation for themselves.
~ Depicted from THE HANDBOOK FOR LIFE'S JOURNEY : On The Three Poisons - How to Handle Desire