My vet said that when they half close their eyes like that, they are content and happy. I call them "bunny eyes" because when they do that, you just think they are the cutest, fluffiest thing alive! lol
Mostly mating behaviors, nesting, etc. Most of it is intuitive for folks like you who breed. But there is some things about eye-blazing vs. eye-flashing, etc. My vet also gave me this link: [link] for basic stuff. And this one is interesting too: [link] Again, most of it you will already know, but they explain behavioral things in such a way that puts together a bigger picture. The last link is for parrots in general, but since lovies are a type of parrot, then most of it will apply.
Eye blazing is used mostly in courtship of parrot species. They can control the size of the iris of their eyes (the only animal that can), and they expand their iris and keep it large to show interest to the potential mate.
Eye flashing (also called eye pinning) is where they rapidly expand then contract their iris. They do this usually for communication and it tends to show they are excited about something or interested in something. The excitement can be something good, like enjoying being pet by its owner; or something bad, like when the bird is guarding its territory or favorite toy. For a good example of happy eye flashing, look at this bird's eyes: [link] And this one is being chastised for refusing to go to bed, but he's trying to charm his owner by eye flashing: [link]
Pocky's "bunny eyes", as I call them is only a form of eye flashing. The only difference is that when they half close their eyelids to do it, they are showing pure pleasure and happiness. If he left his eyelid open, then it could be pleasure or warning. If you could see his iris when he half closes his lids, you would see it wide open!
Since lovies have such dark eyes, it is harder to see by us but apparently they can see it very clearly from eachother. Fascinating stuff!