Crying is a natural part of our human package. As babies, we cried to communicate needs for food and care. As adults, we cry less frequently and about different matters. And yes, even the manliest of men are susceptible to this condition - if the beginning of Up doesn't wet the eyes, just try slicing an onion for a few minutes.
Not all tears are the same. As it turns out, there are 3 different types of tears: basal, reflex, and emotional. Though all tears are formed by the lacrimal gland, each type is activated by different stimuli, and each function slightly differently. Basal tears are secreted daily to keep your cornea moistened and your eye from drying out. Reflex tears are stimulated by irritants, such as those found in onions and garlic. As reflex tears are mostly water, they function to dilute the irritants from your eyes so as to not impede your vision.
And lastly, emotional tears are ones that are activated by the limbic system and the hypothalamus as they process your intense happiness or sadness to an event. These tears actually contain the hormones adrenocorticotropic hormones (ACTH) and leucin-enkephalin, which is why a good cry can feel so cathartic sometimes. Unlike the basal and reflext tear types, scientists know less about why we cry for emotional reasons, though some hypothesize that it's one way for us to relieve stress.