Sri Sri Ravi Shankar:
This is a very deep subject. If everything was already predestined, then what is the need to do anything? Then doing Sadhana, chanting mantras or offering worship to God would all be useless activities.
See, when it comes to animals and other living creatures, then everything is indeed pre-decided (by Nature). But in the case of human beings, some things are predestined by destiny, but there is some freedom of choice too. You are gifted with Viveka (discretion) to distinguish between right and wrong.
See, birds and animals do not acquire the fruits (merits or sins) of their actions, since all their actions and activities are programmed in their DNA itself. But in the case of human beings, you have the freedom to choose how to act, and you also have the ability to distinguish one thing from another. So you can change the way things happen as per your wish, that is why human life is well suited for attaining liberation.
So, despite things being predestined in life, you have the ability and free choice to either endure what happens, or change whatever happens in life. Human life is a mixture of things that are predestined, and free will.