Disclaimer: Doctor Who and its characters are the property of the British Broadcasting Corporation. Not mine.
Note: Scientific explanation taken almost straight from the Wikipedia page on the subject.
"Why do humanoids cry?" Tepeth asked.
The Doctor turned to the spider-like creature. Garashanites did not have eyeballs; their eyes were light sensory strands. Nearby, a woman was comforting her weeping sister.
Tepeth spoke softly. "I understand basal and reflex tears. Yet I've never understand why humanoids' tear ducts react to physical pain, or even emotional stress."
"The parasympathetic branch of the autonomic system controls lacrimal glands via neurotransmitter acetylcholine through both nicotinic and muscarinic receptors. Receptors activated: lacrimal is gland stimulated: produces tears."
Tepeth nodded, "Really? I always assumed it was more need for expression. Never mind; thank you."