What emotion does Charlie Gordon experience for the first time with Mrs. Kinnian?

After his operation, Charlie Gordon finds himself falling in love with Miss Kinnian. Before the operation, she was simply an authority figure to him. In his childlike innocence, he saw her as similar to a parent. He admired her and was flattered that someone so important found potential in him, but he never assumed any equality with her. She was above and he was below.
After the operation, he perceives her in a more nuanced way, as an equal and as a human being, not just an authority figure. He invites her to dinner with him and notes,

I don't understand why I never noticed how beautiful Miss Kinnian really is. She has brown eyes and feathery brown hair that comes to the top of her neck. She's only thirty-four! I think from the beginning I had the feeling that she was an unreachable genius-and very, very old. Now, every time I see her she grows younger and more lovely.

He ends the diary entry by stating flatly

I'm in love with Miss Kinnian.

Interestingly, Charlie's ability to love is connected to his ability to think and analyze. As he grows in self-confidence, he develops his capacity for love.