A study by Martin Seligman considered the father of modern positive psychology and the creator of the idea of learned optimism, showed that insurance agents who are optimists outsell their pessimist counterparts by 8 percent in their first year and 31 percent in their second year. ( Search Inside Yourself)
Being optimistic is something related also being stoic. If something is under your control, you should always be optimistic. Because you will be able to figure out what you need to fix or change it.
If it is not under your control, there is no need to being pessimistic. You should be aware of the situation and move on. Don’t kill yourself something you have no power at all.
People like around an optimistic crowd. That energy is definitely key to build or start something from scratch. Send good energy in order to receive good ones.