Generally, when you ask a wh-question (that is, a question beginning with "who," "what," "when," "where," "why," or "how"), you must use subject–auxiliary inversion. By "must," I mean that your question will sound very strange if you don't.
There are are a couple of exceptions:
- If the question word is (or is part of) the subject of the sentence, it remains at the beginning instead of being moved after an auxiliary. For example, we ask "Who framed Roger Rabbit," not "Did who frame Roger Rabbit?"
- Some dialects sometimes don't do subject–auxiliary inversion (but I'm not familiar with these dialects or what their rules are). Edwin gives the example "Why they ain't growing?"
So, you have to ask "Why are you laughing," not "Why you're laughing?" Likewise, it would be incorrect to ask "How this happens?" (which should be "How does this happen?") or "When he will arrive?" (which should be "When will he arrive?").
[I]n the spoken language, since we have intonation I thought it might be unnecessary to emphasize on being totally grammatically correct for every sentence I'm saying.
Well, it's not necessary to be totally grammatically correct in speech. But in my experience, native English speakers would never ask "Why you're laughing," not even in the most informal of situations. For that reason, if you speak like that, you'll sound like a foreigner.
What a native English speaker would do, though—in my part of the United States, at least—is leave out the verb completely, and ask, "Why ya laughin'?" (IPA: /waɪ jə ˈlæfɪ n/) Or, even more likely, they'd ask "Whatcha laughin' at?" /ˈwʌtʃə ˈlæfɪ n æt/ or "Whatcha laughin' about?" /ˈwʌtʃə ˈlæfɪ n əˈbaʊt/.