A memory is formed through encoding (capturing the input), consolidation (linking neurons into a stable pattern), storage (holding that pattern over time), and retrieval (reactivating it). A failure at any one stage, not a single faculty called 'memory,' explains why something is forgotten.
You cannot remember what you never encoded, and encoding requires paying attention in the moment. Most everyday forgetting (where the keys are, whether you locked the door) is not memory failure but a failure to attend, because the action was done on autopilot.
Skills like typing, biking, or playing piano are stored as procedural memory, a separate system from the conscious memory for facts and events. This is why someone can execute a complex skill flawlessly while being unable to describe the steps, and why these memories resist the decay that affects conscious recall.
Emotionally charged or surprising events trigger the amygdala and stress chemistry that tag the experience as significant, carving a deeper, more durable memory. This is why flashbulb memories of dramatic moments feel vivid, even though the vividness does not guarantee the details are accurate.
The brain is built to discard most of what it takes in, because retaining everything would make generalizing, prioritizing, and functioning impossible. Normal forgetting is therefore an adaptive filter, and expecting total recall misunderstands what memory is for.
Momentarily forgetting a name, word, or where you parked is normal aging, because retrieval slows as the brain accumulates information. The warning sign is not forgetting an event but forgetting that the event happened at all, or losing the ability to perform familiar sequences, which points to disease rather than ordinary aging.