How come the roof of my mouth is sore?

It depends: This could be due to trauma or irritation or infection. Somtimes rumors can cause soreness. The only way to diagnose is to see your physician or dentist.

Ulcer: Can occur due to trauma or burning from hot foods.

Soreness: I suggest making an appointment with a dentist or an oral surgeon. The most common cause is burn (from hot food). Fungal infection can cause soreness. Salivary gland tumors (benign, malignant) can become painful.