Join Date: Oct 2010
Location: Victoria, Australia
Most likely because in every animal there exists the drive to procreate and building a bubblenest is a step in this process.
I do think that a betta male that is healthy and comfortable in his environment is more likely to build a nest than one that is not. However, I have seen fish in dirty cups working on nests so I think it's more an instinctual behaviour than a definite indication of 'happiness'.