Just for example, seeing yourself in an airplane and then the engine suddenly becomes crazy. What would you do if your pilot suddenly say "Oh my God, our plane will crush, our plane will crush!" or just the other way around being super optimistic "We will be save, our plane will just be fine!"
Just thinking of the day to day lifestyle that you have. Will you choose the first pilot that keeps on saying negative? or Mr. Save our Lives part?! "Then the one who gives the choice of optimistic decisions will be fine. It probably won't offer any guarantee but it will give you the best opportunity to feel somewhat good.
People who keeps on losing are those who think of the impossible things till everything they view in life are impossible. The optimistic person keeps on thinking the possible things. While focusing on all possibilities, this person make all things possible too.
"Be happy" You ought to be happy with your life, there's only one and we cannot borrow one more again. But, how do you define being happy?! For me it does not mean "to only smile, laugh or joke around" It means be free of stress, live your life just the way your like it to be.
My cousin once told me "Your lucky enough to live one more day, so give yourself a chance to become happy and live for you and love ones as well for God's glory too."