Please think over your question for a while and ask yourselves what sort of answer you would find helpful. When you have decided on that, tell us what you concluded. I am sure you don't want silly answers like: "Because it wants to get to the other side" or trivial answers like: "Because what goes up must come down." 

Give us a hint. In thinking out the hint you also may be able to understand yur own questions better, and that will be far more useful to you than anything we could tell you.