I've discussed this with Zak... and if you notice, except for a few off cases, all of Ash's Pokemon are Loners or Leaders.

Pikachu - Originally a loner...
Pidgeotto - Loner turned leader.
Caterpie - A loner
Bulbasaur - A Leader
Squritle - Leader of the Squirtle Squad
Charmander - Loner
Primeape - Loner
Krabby - Loner
Snorlax - Loner
Lapras - Loner, not through choice.
Muk - Leader of the Grimers.
Chikorita - Loner
Totodile - Loner
Cyndaquil - Loner
Heracross - Loner...
Noctowl - Odd Loner.
Tauros - Problem there... he's got a heard of them, though one could argue he has the leader of the Tauros battle.
Phanphy, Beedrill, Raticate and Larvitar - Problem there...

SPOILERS

Treeko - Leader of the Warriors for the Treeko group...
Corphish - Loner
Toarkoal - Loner
Tailow - Leader

So as you can see, Ash goes for the Loner or Leader Pokemon... However with Phanphy and Larvitar were from an Eggs, Beedrill and Raticate we didn't really know...

Misty however...
Poliwag - Loner
Horsea - Loner
Corsola - Loner who was hated by its group.
Psyduck - Loner.
But Goldeen, Togepi, Starmie and Staryu we can't really categorize... The waters for a lack of personality, Togepi for hatching.

Tracey caught a "Leader" Scyther.... whilst we don't know about Venonat nor Marril.

So what do you all think? Bit too much of a coincidence?