Why are instance variables in Java always private?

I'm newbie to Java and I'm learning about encapsulation and saw an example where instance variables are declared as private in a class.

http://www.tutorialspoint.com/java/java_encapsulation.htm

I have 2 queries:

1. Why are instance variables private? Why not public?
2. What if instance variables are made public and accessed directly? Do we see any constraints?

Can you explain with an example as to what will go wrong in case the instance variables are declared as public in a class in Java?