Character motivation drives a short-term and a long-term goal. The short term goal is the plot goal which drives the book. What does your character want? What gets in the way (conflict)? The long-term goal can be said to fulfill the question: What does your character need? Whether your character changes or not, if you understand where they start, you can make sure their inner goal is tested in the book, giving them the opportunity to change (or not).
Uncovering your character’s motivations means knowing the answers to questions like: What’s their worst fear? What’s their best moment? If you don’t understand the events that shaped your character or you don’t know that your character was cut from a group of girls when she was sixteen and she spent the rest of high school as a loner, you might not understand or be able to share why she pushes away every friend she might have made.
Taking the time to answer questions like that about your characters will make them more rounded for you which in turn will make them more rounded for the reader.