What is the most important thing to carry with you all the time?

Optimism is the most important thing to keep in my pocket, along with my smartphone.I don’t quite understand the point of pessimism. I am not rude enough to cancel pessimists,they have some valid views and can be interesting to talk to, but draining at the same time. I am pathetically optimistic although completely accepting of the serendipity of life including the, sometimes unpalatable, negative aspects.