A Buddhist-inspired response: "Why" isn't really a helpful way to think about it. We'll never know. The fact is, that's the way life is--for people and animals and insects--all beings are in the same boat. We all cling and desire and so we suffer. We'll never know why life is like that, so working on that question will be fruitless. A more fruitful question, according to the Buddha: what should we do about it?