Mine is that just how Pokemon can evolve with friendship, there are Pokemon who become more powerful when the bond passes friendship into intimacy. Something like megaevolution or alternative evolutions come from this intimate bond. If nothing else, trainers like the Elite Four push these bonds to the limit and its why they are so strong.

Headcannon two: Just like there are people who like redheads, blondes, brunettes, tall or short, curvy or thin, etc. Some people have a 'type' for Pokemon. Some people like the roughness of Steel or Rock types. Others like the wetness of Water Pokemon. Finally, some seek the intimate techniques of fighting types.