Submandibular Sialadenitis: This condition, which involves inflammation of the submandibular gland, is a common cause of chronic submandibular tenderness. It can be due to various factors including obstruction of the salivary duct, infection, or autoimmune disorders.

Submandibular Gland Stones (Sialolithiasis): Stones in the submandibular gland or its duct can cause chronic pain and tenderness due to obstruction and subsequent inflammation.
Bruxism or Temporomandibular Joint (TMJ) Disorder: These conditions can lead to referred pain to the submandibular area due to muscle tension and joint dysfunction.
Dental Problems: Issues such as abscesses, impacted teeth, or periodontal disease can cause pain that radiates to the submandibular region.

Malignancy (e.g., Submandibular Gland Tumors): Although rare, tumors of the submandibular gland can present with chronic tenderness and should be considered to avoid delayed diagnosis.
Lymphoma or Metastatic Disease: Enlarged lymph nodes in the submandibular area can be a sign of lymphoma or metastasis from other cancers, making them critical not to miss.
Infections (e.g., Actinomycosis, Tuberculosis): Uncommon infections can affect the submandibular gland and surrounding tissues, leading to chronic tenderness.

Sjögren's Syndrome: An autoimmune disorder that can affect the salivary glands, leading to chronic inflammation and tenderness.
IgG4-Related Sialadenitis: A rare inflammatory condition that can cause swelling and tenderness of the salivary glands.
Kimura Disease: A rare, chronic inflammatory disorder that can involve the submandibular region and cause lymphadenopathy and eosinophilia.