Ash met Pikachu when he was a child
Ash had always had a connection with Pokemon, And one day Ash got lost in the Forrest And a Pichu Helped Him find his way home.
The next day Ash tried to find the Pichu that helped him But he couldn't.
And for the next couple of weeks Ash went into the Forest to try to find the Pichu.
He never found him.
Years go by and Ash Was at Professor's Oak's Lab Talking with his Pokemon, telling them stories about when he was a child.
And Ash tells them of a kind and helpful Pichu that helped him find his way home after being lost in the Forest.
After he finished telling his story Pikachu ran up to Ash and Started talking to him, telling him that he was the Pichu. (Ash can't understand Pokemon like they are talking english, But Ash has worked out How to understand them)
Ash is happy that his partner is the little Pichu that helped him when he was a child.