What are the factors that influence Grete's behaviour towards Gregor?
At first, when Grete is kind to Gregor, she seems to be motivated by both her love for who he used to be as well as by her desire to be useful to her family. Her parents really only seem to value their children when they contribute, through their work, to the family's livelihood, and Grete hasn't been particularly useful—especially not in comparison to Gregor, who supports the entire family—until now. However, her love for him seems to compel her to try hard to find out what food he likes to eat, to arrange the furniture in a way that will please him, and so on.
After she gets a job and begins to work outside the home, Grete seems to feel a great deal less inclined to help out with Gregor at home; however, at the same time, she doesn't want anyone else to deal with him. Perhaps she has grown somewhat protective of her role as the primary provider of his care because she wants to go on being considered useful in this way. She no longer feels the same sense of pleasure in taking care of Gregor and seems to look on it now as an obligation. Once Gregor is no longer in a position to take care of his family, he loses his value, and Grete seems keenly aware of their role reversal: he has lost value while she has gained. Ultimately, then, it is only obligation that compels her to care what little she does for him in the end.