Does anyone else's parent cry for no reason?

Does anyone else's parent cry for no reason? Some days, minutes or hours my Mom cries and cries. Singing stops them sometimes, but certainly not always. Hugs and reassurance is how I try and stop it, telling her everything is okay, etc. but it really doesnt work well. Sometimes its on and off the entire day, some days not at all. She cant really speak well to tell me whats wrong but I dont think she knows either. Sometimes she is hysterically happy and very funny, I wish that was all the time. Sometimes I get get her to laugh in the middle of a cry too, its strange. I am sure its not for attention because I sit right with her, rubbing her back, telling her I love her, and telling her stories. This has gone on for a very long time now and my patience is really being tested.
Am I alone here with this?