"Why do you wear this dress?" This is almost a direct translation from Chinese. Asking the question in this way is a little clumsy because people usually wear things that they like and are appropriate for the occasion. An alternative way of asking it is, "That's a nice dress. What made you choose it today?"

I would usually say, "It's more casual." or "It's less formal". Since you're directly answering a question, there's no need to include "Because it...". Also, when using "like" to compare things, usually you compare to direct /nouns/ (i.e. a skater, a student), not adjectives or modifiers (incorrect: more casual, more formal, normal)

Why do you wear this dress? - 好 Because I want to make myself look more casual. Because I want it to make me look more casual. Because it looks more casual.