We cry for lots of different reasons. We cry when we have eye infections because tears have anti-microbial properties. We cry when we get irritants in our eyes (like when cutting onions), to flush them out. Babies cry when they need something.
We also cry when we are sad, and sometimes we cry with laughter. Nobody knows for sure why this is, but there are lots of eligible theories. Some scientists believe that crying is a way to inform other people about our emotional state or to tell others that we are in pain. Others believe that crying is to bring our emotional state to our own attention, so that we can deal with our issues. Others still believe that crying is a way of blurring our eyes to stop us from physically attacking people when we are particularly angry about something.