What causes someone to be tired and unmotivated all of the time?
Depression can be one cause. Depression is a disease and is treatable. Could also be a number of other medical reasons.
There could be a variety of reasons such as laziness, depression, drugs/addiction, lack of proper nutrients or eating disorders. There are probably more but either way this behaviour should be addressed for the safety and health of the individual
If you feel tired all the time, unmotivated to do things that you used to enjoy, and/or less social than normal, you may be suffering from clinical depression. Depression is usually marked by feeling these things constantly for an extended period of time. Suicidal thoughts at any time may be a sign of depression. If you have recognized any of these or other symptoms, you should talk to a medical profession about your feelings.
Well the person you are dating might get tired of you and you might get tired of the person your dating! This may lead to you guys liking someone else. Or feeling just a bit attractive to someone else. Your friends or your partners friends might get jealous or upset with all the time you have been spending together. Let's just say LOVE IS HARD. it also sucks.