[Important Note: Spike is in his late teens now]
If you love someone, you let them go. Even if that means dealing with the pain of forgetting and being forgotten.
In the shady nooks of the dark downtown Ponyville slums, a peculiar shop opens. They promise great things. Greatly terrifying things. With a simple spell and a strict procedure, they can completely erase somepony from the mind of their client - taking the ache out of their hearts.
Spike thought it was just a rumor, until Rarity passes him one day and claims she's never met him before in her life.
[Based on the wonderfully sad and thought-provoking movie 'The Eternal Sunshine of A Spotless Mind']