By Andee Tagle , Mika Ellison A lot of us are fickle friends to our optimism. It's easy to relish in the brighter side of things when everything is going your way. But on those other days — your car broke down, your kid is sick (again) — finding a silver lining can feel futile. Regardless of your situation, optimism can always come in handy, says behavioral scientist and psychologist Deepika Chopra, author of The Power of Real Optimism, a book published in March. Unlike pessimism, whose evolutionary purpose was to help humans survive by constantly imagining worst-case scenarios, optimism can be more helpful in solving problems in our modern world, Chopra says. "Optimism is what we need to stay engaged and keep deeply caring," especially during hard times, she says. It keeps us "deeply rooted in reality, but at the same time, holds space for hope." And the best part is that anyone can train to see the glass as half-full, says Chopra. "Optimism is a muscle.…