First let's start off that psychology is not like other fields, as it's often theories/opinions.
That statement is someone's way to describe what they found out to be best for them. Not an axiom for everyone.
And default doesn't mean always, it means that one's general state is happiness. For me, for that statement to make sense, the word "happiness" would be replaced with something like "being glad" (gladness?), as I always feel glad of myself/my life but I see happiness as something more active, like being sad. While I see this gladness as a passive state. But again, that's my personal take.
That statement is someone's way to describe what they found out to be best for them. Not an axiom for everyone.
And default doesn't mean always, it means that one's general state is happiness. For me, for that statement to make sense, the word "happiness" would be replaced with something like "being glad" (gladness?), as I always feel glad of myself/my life but I see happiness as something more active, like being sad. While I see this gladness as a passive state. But again, that's my personal take.