He had been a friend of Alexander since youth (also perhaps lovers). Perhaps they modelled themselves on Achilles and Patroclus. They had fought together may times. he was Alexander’s favourite and this certainly would have aroused jealousy.
He sent messengers to Siwa to see whether Hephaestion should be worshipped as a god. The oracle refused his request and so Alexander had to content himself with declaring Hephaestion a hero to whom sacrifices should be directed.