When there is a desire, the mind is agitated and cannot focus on anything else other than the desire itself. For instance, if one wants to drink a cup of coffee, the thoughts are constantly on how to get the cup of coffee. The thoughts lead to actions that will fulfill the desire. One thought leads to another and the goal is to get the cup of coffee and consume it. Till the desire is fulfilled or replaced by another desire, it will constantly occupy the focus of attention of the mind. Once the cup of coffee is in the hand and a sip of it is consumed, then there is contentment. Yes, the contentment or happiness may be fleeting but it is there because the desire was fulfilled. In that moment of happiness, there is no desire or there is desirelessness (only until another desire arises!). Therefore, it can be concluded that desirelessness is equal to happiness and vice versa.
The goal of every desire is to lead up to this desirelessness (happiness or contentment), which is our True Nature. One experiences this state of happiness or desirelessness every day, but the mind interprets the happiness to be due to the attainment of the object of desire and not to the fact that for that fleeting moment there are no desires (or desirelessness). Because of the misinterpretation of this state of happiness, the mind is constantly chasing desires thereby resulting in a state of constant seeking. The mind gives the impression that someday or sometime, there will be no further desires to be fulfilled and then there will be contentment.
But, desirelessness is actually our True Nature and it is present right here and right now. It is a state of peace and contentment. Once, one realizes that the state of desirelessness is indeed one's True Nature, the seeking stops immediately. It is just a matter of Understanding which leads one to Realization.
...courtesy Advaita Talks blog