To be desirous of something is to want it. It's like being wishful. We’re all desirous of a safe place to sleep, food, and perhaps a little bowl of candy.
When you desire something, you want it. Likewise, being desirous is a state of desire, want, and craving. A thirsty person is desirous of liquid. An eager employee may be desirous of a raise and a promotion. Dogs are often desirous of people food. This word can apply to small, momentary wants or deep, yearning needs.