Think about it this way.

If an event happens to you, you will react to it in a certain way.

If such an event happens in a parallel universe again with you, you will react the exact same way. So there is a set way people react to things.

Now, if we knew exactly what these events were due to no randomness, since there is no chance in your reactions, there would be a set reaction.

A person will always have the same reaction to the same event, so to the events that happens there will be a certain reaction. The only way to change this reacton and what you do, is to change the events themselves.

Apart from that, nothing affects these actions from the person.