Basically, having a knack for something means you're either good at it or you enjoy it/you like it. When someone is talking about "labels" in a relationship or romantic sense, a label is the term that defines how serious or casual a relationship is. Example: when you're in a relationship with someone you love you want to label them as your boyfriend. So, in this context :Having gotten the knack for labels early" means : having the desire to define my relationships early on. I hope that helped and makes sense!