Why we can't cross back in time?

I feel like crying
When I remember that the years escaped me
I was still young
My hair was big
My mind was rebel
My dreamer heart had a whole life for ahead

When I see images of that time
May I could see the innocence between the eyes
And a single smile hiding in the joy
Great times that can not return
When I see you now in the time screen
I wonder: why we can't cross back in time?