The way that mams speak to babies is apparently consistent in almost all languages. Children's small children do it when they talk to their dolls, and we do it when we speak to our pets. It must be an evolved trait where we recognize the way to inculcate language into a being that doesn't understand it. What you're doing is guaranteeing that this child won't die. You're guarantein that child will have the equipment, language and speech and prosidy, to win and woo a mate so that those gens then go on. For all the fun and cuteness of it all, it's an extremely urgent survival drama that's taking place under our noses.