From their commencement speech at Stanford, which is all about innovation, empathy and optimism:

Optimism for me isn't a passive expectation that things will get better; it's a conviction that we can make things better – that whatever suffering we see, no matter how bad it is, we can help people if we don't lose hope and we don't look away.

Optimism is a reason to do something. Pessimism is an excuse to do nothing.