Why do share prices move up and down?
Supply and demand.
Because of market participants ( buyers & sellers) willingness to buy/sell at a particular price.
Gravity
Hmm… Gravity makes it go down, then why do prices go up ? As gravity does not work in upward direction.
Any reason for that ? : ) 
When the gravity decreases, the object tends to start floating or get pulled up.
Gravity is directly proportional to stocks. 