Why are you so sad meaning in hindi?

In Hindi, you have two main ways to ask this. The choice depends on who you're talking to.

For a friend, a peer, or someone younger, you would say: तुम इतने उदास क्यों हो? (Tum itne udaas kyon ho?). This is the informal and most common way.

If you are speaking to an elder, a stranger, or someone in a position of authority, you need to be more formal. In that case, you would ask: आप इतने उदास क्यों हैं? (Aap itne udaas kyon hain?).

Using "Aap" shows respect, which is a big deal in the culture.