If he had to explain himself, he supposes it would be because they're so different.
He's been watching her more and more often of late, always surreptitiously; as far as he knows no one's noticed with the exception of the small fire demon, who will not concern himself with such things.
He doesn't quite understand her. Oh, he can predict her; he knows how she thinks and will react to any given situation, but even with his human empathy Kurama will never be able to get inside Botan's skin.
She's irrepressibly bright and bubbly, ever- cheerful and smiling; she always has something to be happy about, even the smallest things, and at times he almost thinks the world shares them with her as an inside joke. Botan appreciates the feel of sunshine on her face and opens her arms to the wind.
Kurama has never been able to share himself so easily.
If the world is Botan's companion thought is Kurama's. His mind is always working; both Shuiichi and Youko are sharp above all else, sharp as the thorns on the rose whip. It is detail that has saved Kurama's life most often, the marriage of exceptional observation and keen intellect; and even if Minamino Shuiichi had not had reason to believe that someone (or something) was after him at almost any given time so quick a brain would never be content sitting idle.
Botan doesn't overthink things. What is, is; Botan simply does what she can. There's Kurama to think, Yusuke to fight, Kuwabara to be chivalrous and Hiei to- well, Hiei to try and not piss off.
There is nothing to be gained in watching her the way he does; nothing to be learned, nothing to take, no rhyme or reason to it.
So why does he find such pleasure in it? Why is seeing her smile enough to lighten his heart? Why would he rather watch her than sakura blossoms?
She is the one thing he will never understand.
He doesn't know why he likes that so much.