Reactive inside module

I am a bioinformatician and just started to learn Shiny.
Can someone explain to me why reactive values has to be passed as input parameters to modules? Why can't we just declare the reactive inside the modules?? It doesn't respond but there is no error message when I declare reactive inside the module.
Thank you

This was a silly question. It was not about the reactive, I just forgot the ns() around the name of my variables in the UI module. Sorry!