Pride can be a wonderful thing, for it leads people and pokemon alike to believe in themselves and their own abilities, and gives them the confidence to strive for greatness. But as in the case of the Piplup, it can easily be taken to dangerous extremes. Every Piplup fancies itself to be an Empoleon, and their interaction with one another is typified by puffing their chests out and seeing who can do so best, or trying in vain (for they are not yet evolved) to subjugate the pokemon in their claimed territory to save themselves the effort of catching their own fish.
Yet Piplup are also too proud to apologize, to patch things over with their rivals, or to accept aid from their friends who wish to help them. In a young trainer's hands, they easily push themselves to extremes, giving pokemon center nurses all sorts of headaches in the process. In the wild – back when they were wild – they would have lifelong feuds with their neighbors starting from things as simple as a single fish caught in disputed territory, which escalated when neither pokemon was willing to make amends or admit wrongdoing unless their neighbor did likewise. Nor was this competition confined along gender lines; male and female Piplup often were far too proud to let love or the propagation of the species get in the way of their pride.
The mystery of Piplup's extinction in the wild is not that it happened, but why it took them so long, given how few Piplup ever set aside their grudges long enough to lay eggs. Perhaps their ancestors tempered their pride with far more wisdom than the few Piplup still around today.