How to find Voltage based on reference nodes?
You can treat the voltage as a height. 
https://www.youtube.com/watch?v=ZSrzvoJcaJ8&amp;g

To measure the voltage we need two points in the space. One of this point is treated as a reference point. We have a very similar situation when we try to measure a height of an object. We need a reference point. The most common reference point is "above mean sea level". But when you measure the height of the table in your house the floor now becomes your reference point.