Males build bubblenests when they're "happy." They don't build them "just" to breed, and often won't build them even when they do breed. Water quality, food, temperature, personality, weather patterns, and being around other males all effect whether or not your betta will build a nest. If you want him to build a nest, change his water, and then give place a mirror, or another male (in a seperate container) where he can see and "spar" with them. This not only gives them good excercise, but it also makes them happier and healthier. My friends across the hall, since I live in a dorm, will often borrow one of my males so that their betta can spar, and usually the betta will build a nest that night. When they see another male, and then the male leaves, they think they won the fight and it gives them a bit of an ego boost I guess.