"They" say a lot of things, a lot of it is just bollocks.
When you boil it down, "destiny" is just defeatism, you don't want to take responsibility for your actions so you claim they were predetermined. Alternatively, it can just be a coping mechanism to accept things that have happened. The only place the concept of destiny has any value in would be in books, in stories.
In reality, life is partially what you make of it and partially circumstance.