I think you know the answer to this, Megan. When you were a little girl, did you hold hands with your parent / siblings? It makes you feel close to the person you're holding hands with and sometimes, it gives you a feeling of security and sometimes, it just because it feels good to hold their hand.
This is usually done at any age as a sign of affection for each other young boys and girls do it as a way to show other people they are an item and hands off to other suitors. Hand holding has endured the test of time and always will