I am always hunting for my keys, usually because I wasn’t paying attention when I put them down, or I was interrupted by something else when I walked in the door, or I put them down on Friday evening and never needed them again until Monday morning.
We really don’t know why we forget things, but there are a number of possible reasons. Firstly, memories are physical, and physical things decay. Neural connections, if not reinforced through rehearsal simply fade away.
Imagine each memory as water captured on a leaf dripping on to the sand, leaving a small indentation. Left alone, the the indentation will fade away as the sun dries up the water and the wind blows away the sand. However, should it rain for days and days the dripping water might bore deeper into the sand. The slight indentation has become a hole which may last for many days.
Another possible reason we forget is because similar memories interfere with remembering. I put the keys down and then moved them later. When it comes time to find them I cant recall where I put them last.
Changes in context can also impact on recall. Perhaps I try and trace my steps, but but the furniture in the room is rearranged. My mental image of the scene when I put down the keys has changed, and so the cues I might have used to aid in remembering where I have put my keys disappeared.