I don't think you can get an answer today that is scientific. More the realm of philosophy still. It probably is a scientific matter, but it remains largely unsolved. The answer is probably somewhere in information theory and the idea of emergent complexity. Certain arrangements of information - the structures that make up things - cause further structuring. It happens at every scale of existence, as far as I can tell. For example at the level of molecules, their properties are determined by the information that describes their atomic configuration. When you have hydrogen and oxygen and combine them into water, the result is more complicated in behaviour, than its elements in isolation. For I suspect eerily similar reasons, stars exist -- highly localized concentrations of entropy in an otherwise apparently empty void. Why isn't the universe a cool uniform gas? Why does entropy localize? Why is there apparent structure?