Sweetgreen (SG -1.51%) is a young restaurant chain that aims to grow in the years ahead, but it's important to know that financial results for this type of business need to be assessed in a particular way. For example, the company's first quarter appears to have been reasonably strong, with the top line expanding by 5.4%.But there's an underlying weakness here that you have to understand before you buy into this salad-focused fast-food concept. Let's take a closer look.What does Sweetgreen do?Sweetgreen is ...