It likes it when people are happy and have the things they want. Any people, but especially the people it sees most clearly, who resonate with its central themes and so are easier for it to perceive and understand.

It can't possibly be that simple, so why is the notebook pretending it is? What's the catch? He considers, once again, the possibility that the notebook is just lying about the whole thing. Assuming it's not, though...

How does that benefit the Spirit?

It's hard to describe in mortal-scale concepts what the Spirit is or where it comes from, but a story you could tell that would be more true than false is that the Spirit is made of people wanting things, and so feels fulfilled when it experiences people getting what they want in kind of the same way that people feel fulfilled when they experience eating food and sleeping and having friends and falling in love.

So maybe something a little like the Gods? He doesn't bother writing that, because he can predict the notebook's answer: it doesn't know enough about this world to make comparisons. 

So you're saying that people getting what they want, especially when they want to be beautiful and special and powerful and feminine, directly helps or feeds the Spirit in some sense? 

Something like that. But more like having friends or falling in love than like eating or sleeping, because the Spirit isn't the kind of thing that has needs, exactly.