Any reason to think that negative feelings are a more effective motivator than positive feelings?

The Happiness Hypothesis, I book I recommend, addresses this. A long string of good decisions/rewards will keep us alive, while a single bad decision/punishment will kill us. E.g. if you do eat that mushroom it might kill you, but if you don't you probably won't starve right away and can probably find something else to eat. Thus avoiding pain is a better policy than pursuing rewards.

However, we're smart and can decide when to override this instinct. For example, my reptile brain just tells me to save all my resources for me and mine. But my more rational mind tells me how much I have in my bank account and that I can afford to help strangers.