Clean/Rebuild Run

As I'm making my application, I've been able to just run my proje开发者_如何转开发ct and it runs with the changes all made. As my project is getting larger, I have to go Clean Solution, Rebuild Solution and Run...Recently I also upgraded "Team System" edition my dad has, because of its testing tools

Why is this happening? is it because of something I've done in my project or is it because of the different version of Visual Studio