Both of my bettas build nests, its just a sign that they're happy. I really dont think i could breed my multicoloured CT boy because he's so aggressive, I think he'd kill a female.
Usually the bettas in petstores seem so 'tired' and kind of 'sad' because theyre in such small containers or cups, and because the water is very rarely changed if at all while theyre there... This new boy (the purple one) was like that, but I recognised his potential and now he's much happier. They seem to LOVE to swim- i used to have casper in a 5L bowl but then upgraded him to a 10L and since then he's blown more bubble nests, swims a lot more, is active, healthy, investigates everything etc. To me thats enough proof that they like their space... so in two weeks my two will be sharing a 40L tank (divided, of course, so they cant see or get to each other)