There friendship really grew alot during the Pokemon Diamond and Pearl generation of the anime.

Okay the one with Pikachu/Piplup is supposed to say LightningPearlShipping.

Pikachu and Piplup's friendship started at the very beginning of the series

Axew and Scraggy are just like brothers. They both obviously care a lot about each other, and they both get upset when the other one is not around. In "Scraggy and the Demanding Gothita" Axew had a nightmare about Ash losing the battle he had against Catherine, and as a result, Scraggy would've had to leave with her, and then when Axew woke up and saw Scraggy and Gothita were both gone he became even more frightened. In the episode "Battling the Leaf Thieves" Scraggy woke up from his nap and noticed Axew was missing and he began screaming as a way to warn the others. In "The Path that Leads to Goodbye" Scraggy wondered where Iris and Axew were and when Ash explained to him that Iris and Axew were going to travel by themselves, Scraggy became sad, and Cilan pointed out to Ash that Scraggy and Axew are best friends. Later on in the episode at night when everyone was going to go to bed, Scraggy and all of Ash's other Pokémon were looking out worried about Iris because it was raining pointed this out to Ash, and Scraggy turned to Ash and nodded his head.

Because Buneary was the one always flirting with Ash. Also, Pikachu and Piplup were both boys. I'm not being homophobic, I'm just saying that there aren't any gay couples in Pokemon.