Could you explain why do i have dry mouth all the time?

See details: Assuming you are not on any meds that could cause it, sjogren's syndrome is a possible reason for the dry mouth. See a rheumatologist.

Call doctor: It could be medications you're taking. If no meds, then you need to see a doctor to have salivary glands evaluated to see if they are functioning properly. There are some medications that can improve salivary flow. Also need to be checked if you have any systemic problems that are causing the dry mouth. Please call your doctor for an exam soon.

Medications: The most common reason for dry mouth is from medications. Talk to your doctor or pharmacist and ask if your medications cause dry mouth. Other causes include mouth breathing, hormonal, salivary diseases and aging. See your dentist regularly as dry mouth cam cause severe tooth decay.

Also Sjögren's : Agree with the previous answer. If the eyes are dry too and/or there is a history of autoimmune disease, also consider sjögren's syndrome.