Why didn't Eren Yeager saw the Key before in Attack on Titans?

Question

In the first season of Attack on Titan, Eren remember something his dad told him about the cellar under their house, when he saw the key, and he transform into a Titan and stopped a canon fire.

We first saw that key with Eren's dad. While he was leaving he said to Eren something like "Be good and you will get this when I get back". Then when the Titans attacked and Eren went to Wall Rose we see the Key in his neck.

I wondered how it got there, but the flashback scene suggest that his dad erases his memory.

But I was wondering, the key was with Eren for the past 5 or so years. If a look at the key would make him remember everything, why didn't it happen during that time?

I am now half way to season 1, and haven't read the manga so if spoilers are there please put a spoiler tag

Answer 1

He is sure to have seen it before, but like most things, he doesn't just automatically remember everything about an object just by looking at it, especially when he has amnesia in regards to those memories. People set alarms to remind them to do things all the time, but there is never a guarantee that the alarm going off will actually remind them to do the task.

Two potential ideas come into play here then:

1. The amnesia is strong, and its only broken when he shifted into a titan. By doing so, the memories were no longer forgotten, and so the key could then remind him.
2. The amnesia is like most amnesia, strong enough that it takes time or extraordinary events to break it. Erin had both. After years, he suddenly wakes up after believing he died, with severed limbs regrown, and cannons pointed at him with people asking if he is a Human or a Titan. That is an incredible stress, adrenaline rush, and also directly related to the memories forgotten around the key. Upon seeing his fathers key after thinking about him being a Titan, he then remembers his father talking about the key and titans, as well as the drug that was injected into him, which is what caused the amnesia to begin with.

It could be a combination of both as well. Regardless, the Drug caused the amnesia, for reasons that are probably spoilers for this question. That amnesia was clearly enough that just seeing the key was not enough to remember.