system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do we tear up when we laugh for a long time?
gpt
It is because the drainage (lacrimal) duct is compressed from us screwing up our face, therefore your normal amount of lacrimal (eye) fluid cannot drain away and thus it overflows onto your face. This duct goes from the inner eye to the nose (thus why you get a runny nose when you cry). 


In reply to the hormonal response, tears do contain endorphins that make you happier.