You're a soul that wants to choose a body.
Every body has some ailment, the soul must accept that. None of them is perfectly healthy. But some suffer from a headache, some have tootaches and some have cancer. A healthier body and mind will heighten the possibility of a better quality of life, happiness and ability to achieve more.
A soul without a body feels discomfort like a person in winter with little clothes. So the soul has the incentive to hurry. But there's a tradeoff - should it wait in the cold to find the health it desires or should it quickly choose one and risk a life of agony.
Everything is a possibility: the healthy might fall greatly ill the next day, and the dying might become a runner. But you, the soul, knows the body it wants when it sees it. Don't get confused, the soul will feel it strongly when it has the orders to enter a body.