Why do women lose weight for men?

It's all about being attractive and having more "options." There are tons of reasons for why a woman would lose weight, but you're specifically asking about losing weight for men.

If you are a heavy woman, in the American culture (I can only really speak of that behalf) it's not always considered desirable. This doesn't apply to everyone- personal preference- but think about it this way...Nobody wants to be the last pick in Gym class...and it doesn't change ever.

Being thin and physically attractive does matter. You're not judged on your personality the first time a guy or gal sets eyes on you. You can claim the latter...but it's just not true.

So...long story short: Women lose weight to be considered more desirable and have more options in a mate- (or whatever they are looking for.)