Lupin has some fairly deep abandonment issues, and a lot of them stem from her older brother, who slowly but surely began to forget and ignore his younger sister. These are the main reasons she has such an issue trusting people when they say they care or love her. She's incredibly skeptical on their word, and even to an extent, their actions, largely due to her expectancy for them to one day walk out of her life without a word, or simply give up on her. So far, a fair amount of people have proven her right, whether it was willing or not. This also partly explains why she has such an inferiority complex and believes herself not worth anyone's time, nor their energy, to put any extensive investment into. But that's a whole 'nother ball game to dig into.