Allergic Reaction: This is often the most common cause of swelling under the eye, typically resulting from an allergic response to substances like pollen, dust, or certain products. The swelling is usually accompanied by itching, redness, and sometimes tearing.

Fluid Retention (Edema): This can be due to various factors such as hormonal changes, high salt intake, or poor circulation. It often presents as puffiness under the eyes.
Sinus Infection: Infections of the sinuses can cause swelling and puffiness under the eyes due to the proximity of the sinuses to the orbital area.
Fatigue: Lack of sleep or chronic fatigue can lead to puffiness under the eyes due to increased fluid retention and decreased circulation.
Aging: As people age, the skin under the eyes can become thinner and less elastic, leading to a more pronounced appearance of puffiness or swelling.

Orbital Cellulitis: A serious infection of the tissues surrounding the eye, which can lead to severe complications if not promptly treated. Symptoms include swelling, redness, pain, and decreased mobility of the eye.
Thyroid Eye Disease: Associated with Graves' disease, this condition can cause swelling of the tissues around the eye, including the eyelids and the orbit.
Angioedema: A condition characterized by the rapid swelling of the dermis, subcutaneous tissue, mucosa, and submucosal tissues. It can be life-threatening if it involves the airways.

Wegener's Granulomatosis: A rare autoimmune disorder that can cause inflammation of the blood vessels, including those around the eyes, leading to swelling and other symptoms.
Sarcoidosis: A condition in which inflammatory cells form lumps known as granulomas in various parts of the body, including the eyes, which can cause swelling under the eyes.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening condition involving a blood clot in the cavernous sinus, which can cause swelling and other symptoms around the eye.