Śrīmatī is the only ocean of anurāga.

ānurāga means mahā-bhāva.
ānurāga is ever-fresh and ever-astonishing.

Śrīmatī thinks Kṛṣṇa’s sweetness is ever fresh and She always has more taste for His sweetness, so She always relishes it.

Śrīla Rūpa Gosvāmī writes in His ‘Ujjvala Nilamaṇi’ (Sthāyibhāva 104):

ko’yam kṛṣṇa iti vyudasyati dhṛtim yas taṇvi karṇaṁ viśan
rāgāndhe kim idaṁ sadaiva bhavati tasyorasi krīḍati
hāsyaṁ mā kuru mohite tvam adhunā nyastāsya haste mayā
satyaṁ satyam asau dṛg aṅganam agād adyaiva vidyun nibhaḥ

One day Śrīmatī Rādhikā and Her friends headed by Lalitā met Śrī Kṛṣṇa.
Kṛṣṇa became overwhelmed by Rādhikā’s mādana rasa and just to joke with Him, Śrīmatī went to see Her girlfriends in another bower. When the sakhīs there pronounced Kṛṣṇa’s name, Śrī Rādhā told Lalitā:
“O Kṛśodari (slender girl)! Who is that, whose name you are pronouncing?”
Lalitā said: “Whoever it is, what’s that to You?”
Śrī Rādhā said: “Sakhi! I have to know, because I am worried about how to keep My vow as a chaste housewife after hearing that name!”
Lalitā replied: “O Rāgāndhe (Girl blinded by love)! But You are always playing at His chest!”
Śrī Rādhā said: “Sakhi! Are You joking with Me? I have never even met this boy!”
Lalitā said: “O Mohite (bewildered girl)! Has Your knowledge been covered over by intense desire? I have just handed You over to Him!”
Śrī Rādhikā then remembers and says: “Sakhi! You speak the truth! In this lifetime He has appeared before My eyes today for one second, just like a lightning-flash!”

This example shows how anurāga makes the anurāgavati (passionate girl) ever more thirsty after Kṛṣṇa’s great and astonishing sweetness and thus makes Her relish this sweetness more and more. With the statement that Rādhārāṇī is the only ocean of anurāga rasa is meant that when anurāga increases, it becomes mahā-bhāva and that when mahā-bhāva increases it is called mādana mahā-bhāva.

Śrī Rādhārāṇī is the only One who has this mādana mahābhāva and She is the fountainhead of all mahā-bhāva, just as the ocean is the fountainhead of all the rivers and ponds.
The other gopīs are Her phalanx who help Her to complete Kṛṣṇa’s relishing of transcendental mellows.

Just as milk becomes thick kṣīra (cream) when it is boiled down, anurāga turns into mahā-bhāva when it becomes ‘thick’ (intensified). Therefore the anurāga-ocean is compared to the sixth cosmic ocean, the ocean of milk.

tika to Radha Rasa Sudhanidhi, verse 18