around here we say bubblenesting equals happiness, but like others have said, what it really means is that the betta is feeling healthy and secure enough to claim his space as a territory and is willing both to attempt to lure in females for spawning, and to defend his territory and offspring (both of which he would need to be at his top game for).
I agree with jollysue when she says this is probably as close to the feeling of happiness that we know that bettas - and likely, other fish - can experience. Their ultimate purpose in life is to make more of themselves and their ambitions don't really exceed that.