Separation anxiety is a normal emotional stage of development that starts when babies begin to understand that things and people exist even when they're not present – something called "object permanence."
At certain stages, most babies or toddlers will show true anxiety and be upset at the prospect – or reality – of being separated from a parent. If you think about separation anxiety in evolutionary terms, it makes sense: A defenseless baby would naturally get upset at being separated from the person who protects and cares for him.
The good news is that separation anxiety will pass and you can take steps to make it more manageable. And in the meantime, enjoy the sweetness of knowing that to your child, you're number one.