Forgetting is actually a natural process and follows a normal and expected curve. Our minds are meant to forget certain information when it is no longer relevant to our lives. So our brains allow us to forget information that is no longer relevant to us.
Memory is retained depending upon the strength of the memory and the time that has elapsed since forming that memory. Stronger memories are kept for a longer period of time, and more recent memories are remembered more easily than distant memories from our past. The forgetting curve shows that we tend to forget about half of all information that we learn in just a matter of a few weeks or even days if we do not consciously review it and attempt to retain those memories longer.
The meaningfulness of the material is also related to how quickly we forget new material we have learned. We are far more likely to forget things that are not very relevant and meaningful to our day-to-day lives than those which are an important piece of our communities and ourselves. For example, although we may not consciously try to remember our wedding day we will likely retain that memory for a long time due to its meaningfulness, whereas what you ate for breakfast last week Tuesday is probably already forgotten.
Repetition seems to be the key to retaining memories longer. And the intervals between repetition would ideally begin close together (days after the initial material is learned) and then can be repeated at longer and longer intervals until finally you only need to review the material every few years to retain the memories and the information involved.