I doubt they ever even met before!
But, if they have, they would probably be the best of friends.
If you want to learn if they can make a baby, please try the Related Question.
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Because they are best friends.
Pikachu. No question about it.
luigi's best friend is Yoshi and Mario
Ash's first Pokemon was Pikachu, he was suppose to choose from the three originals, Bulbasaur, Charmander and Squirtle, but arrived too late, so he was forced to choose from a just currently caught and unstable, Pikachu, which now is his best friend in every season.
They are both 'good guys' in the same series, so presumably, yes.