Most healthy males will keep at least a small bubblenest at all times. But then, most males are always ready to breed, too, so that doesn't say much. Changes in the weather, putting two males next to each others, or putting a female in sight are all reasons for them to create/add to a bubblenest, but largely, bubblenests are just a sign that a betta is happy and healthy. Because of this, if you ever want to buy a betta from your LFS
, a good way to find a good betta is to try and pick one that has blown some bubbles in its cup.
Hope that helped!