Memory is a very interesting part of us. Neuroscience researchers may tell us that our neurons (long cells in our brain and bodies responsible for transmitting thoughts and regulating our actions) form networks with one another to process what we see, hear, taste, touch, smell, etc. into working memory, and short- and long-term memory. Short-term memory refers to the memory that holds a small amount of information, such as a phone number. Working memory refers to the capacity to hold limited information and to process that information, and an example of this type of memory is remembering the numbers and procedures in a particular math problem as you are solving it. After solving that problem, you will most likely not remember anything about that specific problem because you're no longer working with those numbers and procedures. Long-term memory refers to (according to one model of cognitive science) a state in which information is held in memory indefinitely, such that we can recall the information months or years from the moment at which the information was received.

Long-term memory is not automatic; long-term memory of a piece of information or a process usually requires repetition. When we read, our short-term memory is active, and depending on the purpose of the reading we are doing, our working memory is also likely active most of the time. However, if we do not deem the information important enough, we will not re-read or rehearse the information and we will therefore not form any long-term memories of what we read. This is part of the reason that we forget magazine covers at grocery checkouts or plots of the novels we read for school, but we may choose not to forget the plot of the new movie we just watched. To put it another way, forgetting is natural, but remembering takes work.

Our brains forget things because of a process called working memory. There is short term memory and long term memory. When we read something, like the way you’re reading these words right now, it goes into working memory, which decides if it’s important. If we interact with the material or our brain thinks we need it for the future, it gets stored into long term memory.

Working memory is important because it’s kind of like a house-cleaner. We don’t need to store every bit and piece of information we see or hear because there just isn’t enough space in our brain. Instead, working memory decides that we don’t need to remember every street name we see, but we should remember what street name we live on because that’s important for the future, and it stores it in long term memory. If you want to remember what you read, however, you can increase your chances of remembering it by interacting with it, rereading it, taking notes on it, or rewriting it in your own words. That’s why studying and interacting with the material in school can help you recall it on the test!

From what I was able to gather from a quick search, this is an unanswered question in psychology. Among the possibilities are (1) cues that trigger memories are lost or no longer interacted with; (2) memories are interfered with by other memories; (3) biochemical traces that trigger memories are degraded with time. Note that none of these are exclusive with one-another!

Think how crowded our brains would be, if we remembered everything we read. And would we be able to sort the useful stuff from the stuff that's not useful? It's a nuisance, forgetting things when one wants to remember them; but there's probably a benefit to this, too.