Because...string is a reference type! :laugh:
Have a look at the start of this: Using struct and class - what's that all about?
] it explains teh difference, and what is going on when you assign references. (You might want to skip the last half for the moment, it gets a bit busy for beginners).
Once you've had a good read it will hopefully make sense, and then you can explain exactly what you are trying to do and I'll try to explain how to do it.