I've read some things about this, I even found similar question, but it didn't really answer this. For me it seems that privatizing something only makes my life so much harder when I need to find a private variable in a class to use it elsewhere. So what is would the problem be if everything was public? Would it somehow slow the program itself?
You must consider the maintainability of the code. Accessing all the variables everywhere in your solution is good only if you are the only one in the project and you will be the only one that maintain and use the code. If someone else's entered into project and do completely different things they will be able to access your methods/variables and set the things to unexpected variables. You should think as a OOP design and design your classes like that.