explain how, tough question.

I don't understand how you can smile all day long but cry yourself to sleep at night. How pictures never change but the people in them do. How your best friend can become your worst enemy, or how strange it is when your worst enemy turns into your best friend. How forever turns into a few short months that you'd do almost anything to get back. How can you let go of something you once said you can't live without. How even though you know something is best for you, it just hurts the same. How the people who once wanted to spend every seconds with you, think a few minutes of their time is too much to spare. How people make promises despite knowing how common it is for promises to be broken. How people can erase you from their lives just because it's easier than working things out.