Peptic ulcer disease (most common cause)
Gastritis/esophagitis
Gastric/esophageal varices
Mallory-Weiss tear
Malignancy
Aortoenteric fisulta
Boerhaave
Dieulafoy's lesion
Angiodysplasia
Hemobilia
Hemorrhagic gastritis, EtOH
Celiac disease
Dengue
Other intrabdominal bleeds
Hemorrhagic pancreatitis
Splenic rupture
Subcapsular cavernous hemangiomas
Peliosis hepatis
Hemorrhagic pancreatitis
Splenic rupture
Subcapsular cavernous hemangiomas
Peliosis hepatis
Hemoptysis
Vaginal/Urethra bleeding
ENT bleeding
Dietary (Iron, bismuth, beets)
Swallowed maternal blood (in neonate)
Undifferentiated upper gastrointestinal bleeding
Peptic ulcer disease 
Lower gastrointestinal bleeding
Gastritis/esophagitis
Acute gastric dilatation
Gastric/esophageal varices
Splenic infarction
Mallory-Weiss tear