One of the most iconic rivals in animated history is Pokémon's Gary Oak and Ash Ketchum. From the very first episode of the saga, Gary makes it clear that he is not Ash's friend, but only his competition to become the world's greatest Pokémon master. However, a closer look at that legendary first episode of Pokémon reveals that Gary and Ash are actually more similar than one may have thought, especially considering their decades-long rivalry.
Ash Ketchum wakes up late on the morning that he's set to receive his starter Pokémon and begin his journey to become a Pokémon master. As he rushes down to Professor Oak's lab, he considers the three options available to him: Bulbasaur, Charmander and Squirtle. Although all three Pokémon have merit, Ash eventually decides to choose Squirtle.
However, because Ash overslept that day, nothing is left when he arrives at the lab -- all three starter Pokémon had already been claimed. Outside the lab, Gary mentions how he got the "best one of all." Later, it's revealed that just like Ash, Gary picked Squirtle, both believing at the beginning of their journeys that Squirtle was the most advantageous starter Pokémon.
Compared to Bulbasaur and Charmander, Squirtle is quite versatile and strong against many different types of Pokémon, especially Rock Pokémon, which are notoriously difficult to beat by simple brute strength. It makes sense why both Gary and Ash wanted to claim Squirtle from the get-go -- it would have been an advantage during the early gym leaders of the Indigo League. Its Water-type moves would be powerful against bug, grass, fire and rock Pokémon, setting its partner ahead of the game early on.
This reveals that Gary and Ash had a lot in common quite early on. Their ways of thinking in terms of battle strategy were very similar, and had they not been set to be rivals from the outset, they could have been excellent friends. While their rivalry constantly pushes each of them to better themselves as Pokémon trainers, their drive and absolute love of Pokémon is something they equally share. It's that drive that leads them to the successes both of them achieved.
Even though they see each other as enemies, obstacles to each other's success, Gary Oak and Ash Ketchum had a lot in common at the beginning of their journeys as Pokémon trainers. They were both smart enough to choose Squirtle as their starter, even if Ash lost out on that opportunity and was fated to partner up with Pikachu instead. While Pikachu is the perfect match for Ash, his initial desire for Squirtle ties him to Gary more than he'd probably care to admit.