I've always assumed this is related to relief. As an example, when in a lot of pain, if you receive an injection of morphine or some other painkiller and it quickly takes the excruciating pain away, some are likely to tear up a bit. Granted, that's an extreme example of "tears of relief," but I think it's applicable.
If you have a full bladder and you rush off to empty it, you experience the relief of, well, relieving yourself, and I think it makes sense for some people to sometimes experience "tears of relief" during that.
To test this theory, see if you shed tears more readily when your bladder is really full and you really had to go than you do when you didn't really have to go but you went anyway.View Thread