Why?

Why do we do things which we do? Why do we behave in certain way? Isn’t everything already written in our destiny? Even if you change yourself, does it guarantee that it will change what is in store for you? Isn’t sometime being happy is the only option because being sad will not change anything? There’s a time when you believe in something and there comes a time when your belief is shattered to pieces, what you can do at that moment other than cursing your destiny? Isn’t god the supreme of all who has decided what will happen in your life? Can you fight against him? Why is he taking the test of only few? Why are there people who never face any odds throughout the life and why there are those who are against the odds every other second? WHY?