Any event happens due to the effect of interlocking events of the past.
Dasaratha was under a curse to die of Putrashoka because, while hunting in the forest, he had inadvertently caused the death of the young son of an aged, blind couple.
He wanted to marry Kaikeyi, a foreign princess of exotic beauty; due to kama, he promised the father that her son would succeed him as the king -- ignoring the law of primogeniture.
He sent Bharata and Shatrughna to Kaikeyi's father's home. When they were away and without informing Kaikeyi, he made arrangements for the coronation of Rama - ignoring the promise that he had given at the time of his marriage to Kaikeyi. He wanted to avoid a confrontation with her.
He almost succeeded; due to her love and affection for Rama, Kaikeyi at first did not see anything wrong in this. But when her well-wisher and representative of her father - Kooni convinced her, she reminded Dasaratha of the two boons he had granted to her when she had saved his life on the battle-field and demanded that he crown Bharata and send Rama to the forest. Perhaps, we blame Kooni unnecessarily for protescting her ward's interests and doing her duty.
Dasaratha could not proceed with the intended coronation of Rama and, due to his ill-health, age and strong emotions, swooned and fell down. When summoned by Kaikeyi, Rama came and decided that the only dharmic way of dealing with the situation was to accede to Kaikeyi's demands which Dasaratha was bound to concede.
The primary cause of Rama proceeding to the forest appears to be the consequences of Dasaratha's actions - not Kaikeyi or Kooni who were merely agents to execute the Law of Karma.
In the forest also, destiny seems to have guided the handling of the situation involving Soorpanakha. She was teased by Rama to approach Lakshmana who got angry and disfigured her face by cutting her nose. In the normal course, Lakshmana who greatly respected ladies, would have adopted other means to get rid of her as a nuisance. It was because of this incident that Soorpanakha sought revenge, enticed Ravana with an account of the beauty of Sita and persuaded him to abduct her.
When Mareecha came on the scene as the golden deer, Lakshmana could sense that the deer was the demon in disguise; but Sita insisted and Rama chose to follow the galloping animal far into the forest and finally kill it with the disastrous consequences that followed. Sita, a normally respectful person who was aware of the ability of Rama and the fine qualities of Lakshmana, abused him and goaded him to leave his guard-post and proceed to save Rama who she knew was capable of destroying hordes of demons.
If rescuing and recovering Sita was the objective, Rama could have preferred to seek the assistance of the invincible Vali; yet he chose Sugreeva, killed Vali, crossed the Ocean and battled with Ravana.
Every event in Ramayana seems to be tuned towards one single goal: the battle with and Killing of Ravana and his horde of Rakshasas: that was the primary purpose of the Avatar.
But, perhaps, there is an embedded lesson for us in this great story: namely, to educate us on the frailties of the human character and the weakness of the human intellect when it starts acting on its own limited capabilities, however high. That is why after Sita's ordeal by fire in Lanka after Ravana's death, Rama told Brahma: 'I do not know who I am, except that I am the son of Dasaratha, King of Ayodhya.' May be, this is the important message: you are not the limited person that you imagine; you are informed, guided by the illuminating spirit and you should make an effort to realize it.
All the circumstantial events narrated in Ramayana, which do not make sense in a dharmic way, were perhaps intended to tell us forcefully how we suffer due to lack of contact with this superior intelligence which resides in the heart and is always within our reach.
Narayana Bhattathiri in his Narayaneeyam (Canto35 Verse 10), written about 400 years ago, noted the inexplicable situations in Ramayana (like the fire-ordeal of Sita) and observed:
"This human embodiment of Thine is for instructing mankind how too much attachment (Kama) will lead to pangs of separation and how addiction to Dharma (the letter of the Law) will push one to such Adharma as abandonment of innocent ones. Otherwise, it is unimaginable how Thou, who art ever established in the Atman-consciousness, canst ever have any weakness of the mind."