Why do shampoos lose their effect, and you have to change brand?

OK, now you may think I've posted in the wrong section, but I'm a scientist dammit, I'm after a more scientific explanation than I'd get in a more general section. I have a suspicion that chemistry is the right angle to approach from.





Basically, I've always had this experience, whereby a shampoo that you've used for ages suddenly doesn't leave your hair shiny and smooth anymore, and you're left vaguely unsatisfied.





Then, you pick up another brand (I alternate between Pantene and the Aussie stuff) and the new brand gives you shiny silky smooth clean hair again. A couple of months later, the same thing recurs but now the original brand is the one that works better.





Maybe I'm imagining it, but it doesn't feel that way. All I know is it baffles me.Why do shampoos lose their effect, and you have to change brand?
Okay, I have no idea why but I absolutely agree!





I thought it was just because I got bored/used to the smell and texture and I needed something different to really be able to SEE the improvement.