It was a short story about a human who makes contact with an alien species so much advanced emotionally they have developed a variety of terms to describe emotional rapports very accurately. I think it was by Sturgeon.
The protagonist, Jefferson Toms, is a tongue-tied lover:
At last he asked her to marry him. He was willing to admit that he "loved" her—but he refused to expand on it. He explained that a marriage must be founded upon truth or it is doomed from the start. If he cheapened and falsified his emotions at the beginning, what could the future hold for them?
Doris found his sentiments admirable, but refused to marry him.
"You must tell a girl that you love her," she declared. "You have to tell her a hundred times a day, Jefferson, and even then it's not enough."
"But I do love you!" Toms protested. "I mean to say I have an emotion corresponding to—"
"Oh, stop it!"
In this predicament, Toms thought about the Language of Love and went to his professor's office to ask about it.
"We are told," his professor said, "that the race indigenous to Tyana II had a specific and unique language for the expression of sensations of love. To say 'I love you' was unthinkable for Tyanians. They would use a phrase denoting the exact kind and class of love they felt at that specific moment, and used for no other purpose."
Toms nodded, and the professor continued. "Of course, developed with this language was, necessarily, a technique of love-making quite incredible in its perfection. We are told that it made all ordinary techniques seem like the clumsy pawing of a grizzly in heat." The professor coughed in embarrassment.
"It is precisely what I need!" Toms exclaimed.
Our hero makes his way across the galaxy to Tyana II, and finds an instructor to teach him the Language of Love:
Varris was a thorough teacher. In the beginning, with the aid of a portable Semantic Differentiator, he taught Toms to isolate the delicate apprehension one feels in the presence of a to-be-loved person, to detect the subtle tensions that come into being as the potentiality of love draws nearer.
These sensations, Toms learned, must never be spoken of directly, for frankness frightens love. They must be expressed in simile, metaphor, and hyperbole, half-truths and white lies. With these, one creates an atmosphere and lays a foundation for love. And the mind, deceived by its own predisposition, thinks of booming surf and raging sea, mournful black rocks and fields of green corn.
"Nice images," Toms said admiringly.
"Those were samples," Varris told him. "Now you must learn them all."
At last, having mastered the Language of Love, he returns to Earth and Doris:
She opened the door and Toms saw that she was more beautiful than he had remembered, her eyes smoky-gray and misted with tears, her hair the color of a rocket exhaust, her figure slight but sweetly curved. He felt again the lump in his throat and sudden memories of autumn, evening, rain, and candlelight.
"I'm back," he croaked.
"Oh, Jeff," she said, very softly. "Oh, Jeff."
Toms simply stared, unable to say a word.
"It's been so long, Jeff, and I kept wondering if it was all worth it. Now I know."
"Yes, my darling! I waited for you! I'd wait a hundred years, or a thousand! I love you, Jeff!"
She was in his arms.
"Now tell me, Jeff," she said, "Tell me!"
And Toms looked at her, and felt, and sensed, searched his classifications, selected his modifiers, checked and double-checked. And after much searching, and careful selection, and absolute certainty, and allowing for his present state of mind, and not forgetting to take into account climatic conditions, phases of the Moon, wind speed and direction, Sun spots, and other phenomena which have their due effect upon love, he said:
"My dear, I am rather fond of you."
"Jeff! Surely you can say more than that! The Language of Love—"
"The Language is damnably precise," Toms said wretchedly. "I'm sorry, but the phrase 'I am rather fond of you' expresses precisely what I feel."