Although traditionally Psalm 137 is seen as a sad and mournful psalm, through a close examination of the psalm we can view it in a completely different light. Rather than describing the mourning in exile, the psalm may be expressing the mourning as the cause for celebration- by refusing to forget about our status in exile we merited our return to the land of Israel. 

Courtesy of www.tanachstudy.com

Courtesy of www.tanachstudy.com