To dream about having a girlfriend if you don’t already have one can mean that you’re looking for acceptance, love and affection. It can mean that you’ve been lonely or that you’d like to connect more with other people.
If you already have a girlfriend
This usually just means that you’ve been thinking about them a lot during the day. It can also mean that you’ve been reflecting on your relationship with them and how things are going. It’s not always a bad thing, and in fact most of the time dreaming about your girlfriend just means you’re very much in love and can’t stop thinking about them.