Not everyone has a middle name. It is up to the parents to decide if they want their child to have one or not. Some families have special middle names that every boy or girl in the family has to have, it's like a tradition. My family has one. Every boy that is born has to have the middle name 'Picton'. Not sure why, it is just like that!
Usually people have middle names because their parents want their child to be named after a relative in some way. However, some people decide not to give their child middle names and it's totally okay! And some parents give their child more than one middle name. Everyone is different! For example, I have a middle name named after my great-grandma, but my best friend doesn't have one. My friend at school has three middle names!