The natural breeding season for Bettas is first thing in the spring, when the rains come in Thailand and the water level starts to come up, bringing with it lots of fresh, small food. So even in captivity, they get very excited when the storms come, and the barometic pressure changes. It means breeding season to them, and they really respond to it by building their nests. This is similar to when they get a fresh water change, I believe...all that fresh water stimulates their nesting because it resembles the addition of fresh water from spring rains.