When New Moon begins, Bella is alone. Edward has left her, believing his departure would better serve to protect her. He must break her heart, or at least make her believe he no longer wants to be with her, so that she can move on with her life. She, however, wants to hold on to him, but she discovers that she can only vividly recall him to mind when she puts herself in dangerous situations.
In this sense, Bella uses human ingenuity, although misguided, to allow her to retain some connection with Edward, even though the connection rests purely within her mind. Jacob uses the opportunity to rebuild the dirt bikes as a means to further develop a relationship with Bella, in essence creating a kind of relationship that did not exist before.