Shiva was so deeply grieved by the death of Sita that he forgot the basic nature of things, and who Sita really was. When Parvati again manifested Shakti, Shiva was so grieved by the loss of his Ardhangini that he did not notice his Ardhangini was again standing before him. After the death of Sita, Shiva undertook profound Sannyasa, and by Hatha misused renounced Kama. In renouncing Kama, he renounced all his work, his Artha. In renouncing his work, he renounced his duty, his nature, his Dharma. And so Shiva slipped from meditation into sleep.
When Shiva renounced his work, terrible evil arose in the world. Tarak the Asura prosecuted the ancient war between the Asuras and the Devas with vicious success, having obtained a blessing from Brahma that he would only be killed by a son of Shiva. Tarakasura thought that since Shiva would bear no child except with his Ardhangani, and his Ardhangini was dead, and even if she was again manifested Shiva would not see her because of the delusion of his grief, that he would be safe forever.
When the Devas asked Vishnu for help against Tarakasura, Vishnu suggested that they awaken Shiva, and show him his Ardhangini was again by his side. But while most of the Devas could not wake Shiva, those who did failed to help Shiva see that Parvati had manifested Shakti, and was his Ardhangini. All the beings tried to wake Shiva, and show him - but none succeeded. Vishnu knew, though, that one of the Devas had not tried.
Kamadeva is an Avatar of Vishnu. Kamadeva had manifested Vishnu to become the most powerful being in any world. He was victorious against even Brahma, and had defeated Shiva before. But Shiva promised that the next time that Kamadeva attacked Shiva, Shiva would utterly destroy Kamadeva. Knowing the importance of Kama to Artha, and ultimately to Dharma, for the sake of the universe, Kamadeva stayed well away from Shiva.
Now, Kamadeva was the only one who could succeed, and was called upon for a suicide mission. Rationalizing that there would be no use for Kama without Dharma, and that Tarakasura was destroying the Dharma, the Devas persuaded Kamadeva to sacrifice himself. Kamadeva knew that he would be destroyed, so before he left on his mission, he said a goodbye to his wife, Rati. Rati was also his vehicle, and he did not want her also hurt in the battle with Shiva. Rati said she could not live without Kamadeva, for they were one and the same; Kamadeva said this would be the reason he could again return to her. As Sita returned to Shiva, he promised so would he return to Rati.
Kamadeva began his work by first transforming Parvati into Uma, an irresistible beauty. Instructing her to stand before Shiva, Kamadeva prepared himself for the moment of his ending. Kamadeva did not have to sneak up to Shiva, he was so deeply asleep. The instant that Kamadeva struck Shiva, he awoke, and saw Parvati, and loved her. Kamadeva, fully understanding his mission, inspired sexual desire in Shiva, and he instantly copulated with Parvati. Thus, Kartikeya was conceived (incidentally, so was Hanuman - but that is another story). Kartikeya would come to defeat Tarakasura.
Shiva was so very happy that his Ardhangini had returned - and pleased beyond joy by his sons. But when he understood that Kamadeva had attacked him, he remembered his promise with some regret. Kamadeva knelt before Shiva in surrender, but not defeat. To Shiva's surprise, Kamadeva encouraged Shiva to fulfill his promise to destroy him, and all his promises - but to be more careful in making promises in the future: for sometimes a friend is required to harm a friend, as Kamadeva had harmed Shiva to wake him. Sometimes, even lovers fight, and hurt each other. Even in the most intimate ways. Kamadeva professed his love for Shiva, his brother, and called upon his brother to fulfill his promise. Now, it was Shiva's turn to harm Kamadeva: Kamadeva was at the verge of being overcome by pride, and needed to be awakened too. Shiva, ever more grateful to his brother, Kamadeva, was unable to refuse Kamadeva and burnt the brave Kamadeva with his third eye. Shiva utterly incinerated Kamadeva.
There was nothing left of Kamadeva, and Rati grieved profoundly. But Shiva reminded her of what Kamadeva had said to her. Kamadeva would fulfill his promise. Rati was confused - how was this possible? And thus, by profound effort, she understood. She manifested by her love of Kamadeva the Avatar of Kamadeva - in the womb of Krishna's wife Rukmini, as Pradyumma. Vishnu, as Krishna, also missed Kamadeva, and had manifested the Avatar in the womb of Rukmini. Rukmini, too, missed Kamadeva, and manifested Kamadeva in her womb. Rukmini, the wife of Krishna, was also the Avatar of Laxmi; Laxmi is in the same way Rati, the wife of Kamadeva.
But because Shiva had destroyed Kamadeva at Kamadeva's request, there remained some doubt for a long time whether this meant that Kamadeva was truly invincible. Was Krishna a more powerful manifestation of Vishnu than Kamadeva? Krisha had never nor would ever be destroyed. Thus, Kamadeva and Krishna eventually tested their strength against each other in battle, and in mutual victory, completed the Self-Sacrifice.