How does tearing work?
The regulation of tear production (lacrimation) involves reflexes initiated by sensory neurons in the cornea. Corneal pain receptors (nociceptors) and mechanoreceptors detect damaging stimuli and induce tearing to cleanse the ocular surface (and, hence, Shigeta is right as tear gas stimulates pain receptors). Corneal neurons sensitive to drying of the ocular surface also appear to increase tearing. These neurons also respond to cooling and hyperosmotic stimuli (Kurose &amp; Meng, 2013). The tear duct is necessary to drain the tears away from the eye into the nasal cavity (Fig. 1).


Fig. 1. Tear glands and tear ducts. Source: Mayo Foundation for Medical Education and Research.

Strong emotional responses like crying can result in an overflow of tears (epiphora). Epiphora is mediated by the limbic system (the emotional center of the brain), especially the hypothalamus. Such strong parasympathetic stimulation is accompanied by other symptoms, like reddening of the face and convulsive breathing (University of Minnesota, Duluth).

Reference
- Kurose &amp; Meng, J Neurophysiol (2013); 109(10): 2517-22