Sif was a Norse warrior dedicated to Gaia herself and it was inevitable that she would become the lover of Gaia's son, Thor.
Due to her relationship with Thor, Loki and Sif developed a strange relationship. Loki respected her more than the other members of the Norse pantheon and yet berated her constantly for choosing his brother as a mate. Loki was not an intelligent man but he had a quick wit and an ability to see through the lies and bravado of others, he could see what kind of person they truly were. Loki was also a proud and honorable God, something that put him at odds with the other Asgardians. Although the Asgardians were brave and fearless warriors, they had no code of conduct or honor toward their enemies. When met with an enemy the Asgardians could be incredibly cruel and destructive. Cowards and weaklings that stood in their path could be killed instantly, or tortured and raped for weeks, depending on the mood and the spirit of the battle. Loki found their cruelty contemptuous and petty.
In the night of infamy that would become known as the Lokasenna, the Asgardians had just fought against the Gods of Olympus and won a great victory. They had wiped out the Greek's centaurs, chasing them down in their chariots and murdering them in cold blood. Although the Greeks had offended the Asgardians, Loki found the genocide of an entire species to be a disgusting way to retaliate for what was a small insult. That night they drank the Golden Ambrosia, a magical wine made from the golden apples of Asgard and the Ambrosia of the Greeks. It was served in crystal glasses that took centuries to form in the realm of the Ice Giants. Loki was in a foul mood and after two days of revelry he could tolerate no more of their loud, boisterous behavior. The party was hosted by Gymir, a Sea God and it was his liquid servants who served the Asgardians. It was simply too dangerous for flesh and blood creatures to be in the presence of the Aesir during such events. Thor could not attend so Sif attended the event with her half-brother Bragi and his wife Iounn. Sif did not know Loki had arranged it so that he would arrive at the same time and could serve as Sif's date for the three day celebration. To say the Asgardians would engage in debauchery and adultery at these events would be an understatement. The Vanir and elves had also sent emissaries to attend, often for no reason other than to drink and breed with the Aesir.