The definition of what is means to be alive has been interesting scientists for decades. Life, at the simplest level, I think is best defined as chemical reactions that are the product of natural selection. It is evolution via natural selection that makes complex life, like animals.
Some things we just can’t answer and maybe we never will. It is already hard to really understand HOW life first appeared and how it got so complicated, even if we know more or less how it could have happen (through chemical reactions and natural selection, like Claire said), but knowing WHY it’s an even harder question and I don’t think science would ever tell us that. For that sort of question we might just have to find answers outside science or simply feel happy that animals do live and so do we!