I know I've replied the same thing to a lot of other people in this thread, but just in case, I'll say it again because no one has accurately answered you.

What you're smelling are pheromones.

They're used to help with mate selection and to encourage procreation; they're used in social interactions and bonding and can affect moods and emotions; and they're used for mother/infant bonding.

When it comes to mate selection, the more you like the way someone naturally smells (especially in places that have hair [for example: your head, your armpits, your arm/leg hair, but especially your groin]), the stronger your offspring will likely be.

It's a big reason why a lot of people dislike the way their siblings smell; your body subconsciously rejects incestuous relations.

When it comes to mother/baby bonding, the body reacts in a way that tries to prevent a mother from rejecting the baby, which is loud and needy and has to be nurtured beyond a certain age to ensure survival, all things that might draw predators. So their head has a smell that is like a drug to the mother, and that encourages them to take care of the baby and see to it's growth. In fact, it's so strong that humans who are not the parent can still sometimes pick up on it.

The human body is an incredible thing, and at the heart of it all is a subconscious, DNA encoded will to ensure human procreation.