The people on your friend suggestions list on Snapchat are usually on your phone's contact list but have not yet been added to the app. These users are also generally people with who you share interests. In addition, Snapchat algorithms consider how often you interact with someone on Snapchat and how often they interact with you. So, if you're constantly sending snaps to someone and they're replying just as frequently, there's a good chance they'll end up on your friend suggestions list.