This is a good question for which there are several theories and causes. The first is retrieval failure. The concept of retrieval suggests that memories are latent in our minds and can be "retrieved" at any time. However, retrieval can become difficult or impossible due to something known as "decay theory." According to decay theory, every time a memory is created, a new memory trace is created. If the information is not periodically retrieved, it will decay and fade.

There are other theories on this subject, including that of interference. This states that a new memory, particularly if it is similar to an existing memory, can warp and manipulate an already existing memory. This can also work the other way around, and an old memory can manipulate a new one.

Sometimes, forgetting is simply a matter of a memory never being created in the first place. This occurs when information is not clear enough to be processed or the brain is in an altered state.

According to Loftus and Hoffman (1989) in "Misinformation and memory: The creation of memory", the reasons behind failing to remember information are the following:

1. Decay theory- the idea that when people receive input they need to keep it for a specific period of time in order to process it into short or long-term memory, or else the memory will vanish, or decay. This makes retrieval of information very hard.

2. Proactive interference- when an old memory is so ingrained that it makes new memory creation hard. It is a powerful memory overlapping another.

3. Retroactive interference- the opposite of proactive interference; it means that new memories may make you tend to forget old ones. For example, you may change schools, make new friends, and go through new experiences that overlap the old memories from your previous school.

4. Encoding failure- occurs when the input is not clear enough to be processed and, as a result, it does not go either in short or long-term memory. The implication of encoding failure in human communication is that input should be clear, logically-sequenced, and concise for processing to occur.

5. Suppression/repression- is the process of intentionally using interferences or blockages to avoid memory formation. In cases of trauma, the different dynamics prevent memory formation to occur due to consistent interference and disruptive brain processes. In suppression, the individual consciously chooses to forget a memory. In repression, memory is forgotten through trauma and inadvertently.

In all, the reasons behind memory loss or memory failure are both cognitive and physically-based and entirely dependent on social interaction.