All desire can be bottomed out as the attempt to maximise pleasure and minimise displeasure. So you can see how fear can be intimately wrapped up in desire; scared to your not going to get what you want, scared of losing what you have gained, scared of getting what you don't want. Furthermore, desire itself is an unpleasant phenomenon. It causes a sense of lack. And you're right, fulfilling desire won't make you happy for two main reasons - 1) desire cannot be satisfied. Gratifying it may bring pleasure but that reward does not last, does not fully satisfy, does not take away desire completely and 2) gratifying desire actually increases desire. Its a thirst that cannot be quenched.