I can’t answer this question on the spot but I will have my own opinion for this question as I have/will set my own purpose of life. Condition will be similar with everyone else. Still we can have a common answer because we are all human being. We have different form than other things in this nature.
Well the answer is to think rationally. But why would anyone agree with me? Well this is not my random thought, the source for this opinion is “Sophie’s world” a novel about the history of philosophy. In the book, philosophical argument of Aristotle is presented, that says everything in this universe has its own form, and form determines the purpose. Plant, bird, animal, rock… everything has its own form that make them identical. For example, if we thinks about horse, we get a form which is animal that is powerful and good at running. Human beings are formed with higher ability, ability to think rationally which is spark of divine reason.
Personally I don’t have satisfying answer for the question. The standard answer to think rationally, I am going to use it to find out my personal answer, my purpose of life. Now this makes me feel like being in form just like if a horse is ready to run out of stable.