Friends are supposed to bring out the best in you, but then, Green and Red aren't exactly friends.
Around Red, Green is a maniac, a monster. The sullen eleven-year-old who sulks quietly in his grandfather's lab disappears when Red comes near; eaten alive by a brash, hurtful boy who can only communicate in insults and condescending advices.
And really, what can Red say to that? So Red doesn't say anything at all.
But then, that only feeds the fire, because leaving Green to silence is like trying to cage a wild animal, terrified and snarling in the closed space. Green talks louder and louder when Red is near, filling the void where their friendship used to be with nasty words and arrogant boasts to drown out the ridiculous (frightening) notions that flicker through his mind when he's alone with his rival, alone with his own thoughts—Red smiling and laughing, Red following him through Kanto, Red cheering him on during Gym Battles, Red holding his hand, Red in girls' clothes.
There's no room for weakness (ambiguity) in Green's world of researchers and pokémon battles and missing parents, so he fills the gaps in his certainty with poison. If Red makes Green feel lost and confused, then Green has to work harder to ensure that Red is the one left behind while Green soars ahead in the League. If Red's indifference makes Green want to scream and cry, Green has to fight to force out some kind of reaction.
No kindness. No encouragement.
No friendship.
(But still, no amount of toxic influence can stop Green from dreaming of tongues and roaming hands.)