Sallu is Shahrukh's enemy and Shahrukh is Aamir's enemy,so Sallu is Aamir's friend.This seems too simplistic in itself but that holds true for nation's as well.
During the Iran Iraq war since US was never appreciative of Iran's geopolitical adventurism ,US was against Iran.Iran was against Iraq,so US had been hand in gloves with Iraq against Iran.
This was used against the Britishers when some nationalist colluded with Japan and Hitler's Germany for realising India's aspirations.
This is all the prevalent in Indian politics where sleeping with the enemy is acceptable till both have a common enemy.That's why every five years you see different fellows befriending unlikely political hues.
This has its downturn also .When the common enemy disappears friends turn against each other!!