Read the poem "When the Fat Girl Gets Skinny." Create bullet points answering the following questions as if you were a Martian: What is this phenomenon? Why does it happen? Which one line best answers the second question?

While I cannot prepare your bullet points for you, I can certainly assist you with coming up with ideas for them. In terms of answering the questions as if you were a Martian, it sounds like you are supposed to present ideas objectively, almost scientifically, instead of offering more emotional, interpretive explanations. Therefore, when you prepare your bullet points, you will want to answer these questions as though you have never before encountered these ideas, as though this is a totally new-to-you phenomenon on which you're reporting.
This phenomenon is the experience of an overweight girl who loses weight due to an eating disorder. She does lots of incredibly unhealthy things—physically and mentally—as a result of her disorder and her desire to continue to earn praise from her family and her peers. For example, she drinks alcohol, smokes cigarettes, eats in front of the mirror so that she can watch herself (and, theoretically, eat less), skips meals (so often that she, apparently, faints), and obsesses over the scale. How would a Martian describe this? I think a Martian might describe this phenomenon as a kind of self-inflicted torture. This girl ingests substances which are not healthy and do not properly fuel her body, and this practice makes her lose consciousness (this is not a good thing!). She also treats the scale as though it is the most important thing in her life—perhaps to a Martian it would look like she is worshiping a strange little god who only speaks in numbers, especially because she calls it "a shrine" and shrines are typically religious in nature. Likewise, she watches a television show that idealizes women with near-unattainable bodies and calls it her "gospel": again, the religious connotation is clear. Given these word choices, if I were a Martian, I would assume that weight loss is some kind of worshipful activity, that the scale is a god, and that a focus on bodies and physicality is considered to be a way of praying to or pleasing that god.
There are many reasons to explain this phenomenon. The narrator says that she doesn't eat

Because I only feel pretty when I'm hungry.

In other words, the only way this person can feel good about herself is when she is unsatisfied. If she is not hungry, then she doesn't feel good about herself. What would a Martian think? Essentially, I think, that when she is physically satisfied, she becomes emotionally unsatisfied, and the only way she can achieve emotional satisfaction is by embracing physical dissatisfaction. This does sound like an illness, and if this is what her god (the scale) demands of her, then it does not appear to be a very just god.
In addition, the narrator says,

When I lost weight, my mom [was] relieved she could stop worrying about me getting diabetes.

As a Martian, I might conclude that the girl's mother worships this same god. The mother believes that a low weight is the best way to be healthy and devout, even though the girl feels that having a low weight is actually the result of an illness.
Further, the narrator continues,

If you develop an eating disorder when you are already thin tobegin with, you go to the hospital.
If you develop an eating disorder when you are not thin to begin with, you are a success story.
So when I evaporated, of course everyone congratulated me ongetting healthy.

For a Martian, it would likely seem as though illness, to everyone but the overweight girl, looks like health. Ironic, right? People think she's getting healthier, but she is actually growing sicker as a result of the torture she inflicts upon herself in order to please her god.
I think the one line that best explains why this happens is

How could I not fall in love with my illness?

Because of her illness, she can feel pretty. Because of her illness, her mother is relieved. Because of her illness, girls who have never talked to her call her "an inspiration." A Martian might conclude that she loves her illness because it makes her feel worthwhile, valuable, good. She has no choice but to love the thing that makes her pious, makes her a good daughter, makes her a role model. A Martian would perhaps conclude that a society which prizes self-torture as the only way to praise its god and be considered a good citizen is incredibly backward and is, itself, ill.