The answer to ‘why do leaves fall off trees?’ is simple. Without growing new leaves every year, trees wouldn’t be able to make food for themselves and survive the winter.
When leaves are green they turn sunlight into sugar, which the tree ‘feeds’ on. But when the weather gets colder there’s less sunlight, which means less sugar for the tree. So the tree has to start ‘feeding’ on all the other good things in the leaves, to help itself stay strong during the winter.
Because of this, the green in the leaves starts to fade. The leaves might turn red, yellow, orange or brown. The tree has no more use for them, so it lets them fall. And the next year?