system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is processed food "bad" for you?

Sometimes there aren't many other options for a large section of the population, whether due to affordability or other reasons. 

Additionally, how can we make healthier processed food choices?
gpt
When food is "processed" it generally means that it is altered in someway in between its source and when you consume it. Many foods we eat today are "processed". 

Juice is processed by taking fruits and removing the fiber (solid bits). The fiber in fruit is like a natural constraint. eg. it would be tough to eat 10 apples, but easy to drink 10 apples worth of juice. The juice is calorie dense compared to the fruit making it easy to over consume and gain weight. Milk and juices are often pasteurized, pasteurization, the heating of liquids to kill bacteria, being the process they were subjected to.  So its tough to generalize and say all processed food its bad for you. It really depends on the process its self. 

High fructose corn syrup is the result of processed corn products (glucose) that result in the sweet maple like syrup that they put in everything (fructose). Corn syrup isn't necessarily bad for you in small quantities, but it is very calorie dense. So eating large quantities will result in weight gain. 

White bread is also a processed food. They process the wheat by removing the tough outer shell of the wheat where all the nutrients and fiber are found. You end up getting all the calories from carbohydrates without any of the fiber or nutrients, and often people refer to this as "empty calories". 

TL;DR all processed foods aren't necessarily bad for you and it depends on the process and your diet as a whole.

Edit: Clarified a few things.