As a child, i thought: "too happy" made me sad later. at 30, i'm dx'ed: bipolar ii. is it a "healthy" view to keep? are ups & downs always connected?

Yes: In general, your body and mind strive to maintain homeostasis. Therefore, what goes up must come down. I usually tell people that the hypomanic episodes are nice, but you pay for them. Mood stabilizers help maintain this equilibrium. There is good evidence that the more episodes you have, the more frequent the episodes become and the more severe the episodes, the more severe they become.

Not always connected: One of the best ways to understand the ups and down is what is called a stress-diathesis model. Means that stress affects our biology& physiology and stress is an environmental force.And it can so affect our internal states down to gene expression of genes. Genes that produce neurochemicals. And in the balance, is our resiliency. Resiliency; your toolbox of emotional selfsupport. Thus even keel.