I don't catch the drift between the reason for infighting among the houses though.

It is not over power or dominance right?? aren't they basically like brothers and sisters if they are from the same house.

or seniors vs juniors, this a coming of age thing ? like a hit 21 year old ceremonial test (haha) ?

Or is the darker saints faction and Hades involving into the darkness that's fighting Athena's faction ?

Ya that will do for now.. I should be able to keep up so it's a detective conan case. XD