I don't know why God made us different from each other -- but He did, and we should be grateful for it. After all, think how dull life would be if we were all exactly alike!
And you're right: Some people (like your sister) are just naturally optimistic. No matter what happens to them, they almost always react with a smile and a positive outlook. Unfortunately, however, sometimes their "hope" is little more than wishful thinking. Others seem to be naturally pessimistic, always looking on the gloomy side and convinced nothing is ever going to turn out for the good -- even when it clearly will.
But the Bible urges us not to be misled by either of these attitudes, but to have a different outlook. It tells us to find our hope in God -- not in our circumstances, or in our natural optimism or pessimism, but from God. The Bible says, "Why are you downcast, O my soul? Why so disturbed within me? Put your hope in God" (Psalm 42:5).
How is this possible? It happens only when we realize how much God loves us -- a love so deep that His Son was willing to give His life for us. It happens too when we realize that this life is not all, but ahead of us is heaven if we know Christ. Christ gives us lasting hope -- for today and for all eternity. Is your hope in Him?