I feel blue sometimes....and you are right, it comes at me out of left field! I think it is because I am at the point in my life where the kids are grown and starting to live their own lives but now DH and I are dealing with parents who are becoming more *needy* and, sometimes, difficult to deal with. I generally just try to spend some time with friends who are going through the same things I am. It passes.
I do have several friends who are taking *happy pills* and they do seem to help!