Sincerity

And only Aeryon knows why Belarion went red eyed and went into a berserk state. The warriors of the Steppe cares about Belarion like a brother, to the older one, like a father to the younger ones.

This is a feeling of affection, of sincerity. 

Mayeux and Aeryon could not achieve this. It is not because they are inferior to Belarion but because of their position. 

Aeryon is the Chief and as the chief there is a feeling of awe when the warriors of the tribe pledge tehri allegiance to him

When they fight for the Chief, they fight for the tribe, they fight for duty. It is their responsibility to fight for the chief. 

So, the feeling of treating Aeryon like a brother, like a father would not arise.

Mayeux on the other hand is of Roma origin. And even though he is not discriminated, Mayeux himself is not that close with many people of the tribe.