Losing weight for no reason; should I be concerned or happy?

You should happy and. And concern, because some time this agood sign and some times could be bad one..
Unintentional loss. You should tell your md if you keep losing weight unintentionally especially if you loss about 10 percent. Or more of your original body weight as it. Is abnormal. Some evaluation have to be done by your md.
Unexplained weight. Loss is of concern. You know how hard it is to lose weight, and dr ho's answer is reasonable. If you weigh 450 and drop to 400, liklely a good story and you have an explanation. If you dropped 30 lbs in 3 months from 145 to 115, found on routine f/u with prostate cancer, psa <1.0, and you smoke, time for a lung cancer work up, not prosca (an actual scenario thurs.).