Why does my wife not make an effort anymore?

My wife and I have been married since March 2009, and we both used to put a ton of effort into our appearance, for a night out or a meal, but in the last few months, this seems to be totally one-sided. I am finding, that I am still making a load of effort, from ironing a shirt, to wearing a tie and smart jacket, just to go out for food and to see a movie, but she is constantly just ditching any effort at the last minute, putting on some jeans and a t-shirt and throwing her hair up in a bobble. She used to do the same; with lovely dresses and a pair of heels. Never mind the hour she used to spend on doing her hair! This also spills over into our love life.. Why has she stopped putting in the smallest effort, and what can I do to change things, for the better?