We seek stimulation. Fear of boredom, FOMO. But our nervous systems, as you point out, adapt to certain stimulus levels. Some people grow to need constant noise, but I crave quiet and don't even turn on music when I'm reading or writing.

Happiness is a philosophical pursuit, I believe. Learning to see past momentary pleasure and dig into the meaning of your life. Excitement can be enjoyable (or terrifying), but it distracts us from the contemplation of what matters.