We can see Rama's love for Sita in him saving his wife from Ravana and other things that point to Rama's love of Sita, so why is this so? Why did Rama love Sita so much?
There are probably many explanations for this.
From the marriage of Sita and Rama, (Ramayana 1.73)
Shri Sita, adorned with jewels, took her seat by the sacred fire opposite Shri Ramacandra. King Janaka, addressing the Son of Raghu, said: “O Rama, from to-day my daughter Sita will be your companion in virtue. Accept her, O Prince, and take her hand in thine. This fortunate princess, faithful and tender, will constantly attend you, following you like a shadow, in loving obedience. May you both be happy.”
Rama, a virtuous king, would have done the same to anyone who treated him with such kindness and faith; he would have attended to the needs of Sita because she attended to his own needs.
Here is a line where Rama describes Sita when she has been abducted, and we see that Rama is also quite taken by her beauty (3.60):
O Tiger, if you have seen my gentle spouse, whose countenance resembled the moon in radiance, then tell me fearlessly.
“Why art you hiding, O My Beloved? I see you, O Lotus-eyed One! Do not conceal thyself amidst the trees without replying! Stay! Stay! O Princess of lovely Limbs, hast you no pity for me? Why dost you mock me? It is not your nature to yield thyself to this folly, O Lady of Fair Complexion, it is vain for you to fly me, your yellow sari renders you easily distinguishable, I have seen you! Stay, if you have any love for me I Alas! It is not she—my Sita of gracious smiles! Without doubt, she has perished since my grief leaves her unmoved!
Also, Sita is an avatar of Lakshmi:
...followed by Sita, who is Lakshmi in another form. (2.44)
And we know that Vishnu loves Lakshmi:
Viṣṇu did not kill the demon out of his love for Lakṣmī (Padma Purana 6.7)
So, there are at least three reasons:
- Sita was faithful
- Sita was beautiful
- Sita was a form of Lakshmi, and Vishnu loved Lakshmi, so of course Rama would love Sita as well