Because your MIND knows the truth, & won't ALLOW u to forget it
Give it time
Most of the time, when a person fakes a smile, he or she only uses those muscles in the bottom of one's face. They are forced, without smiling with crinkled eyes as well!
What happens, is that your forced muscle movements (combined with your non-sincere top facial lack of movement) creates pain in your face.
If you are surprised and/or laugh and smile spontaneously (the old-fashioned way), you will notice this is not painful at all!! Why not? Because the muscle movements are "reactions" to your positive emotions, and the muscles are not "forced"!!!
Hope this helps. If you want to see what fake smiles look like -- just look at posed wedding photos -- group shots!!! The guests are just smiling because the photographer asks them to smile -- while the Bride and Groom are genuinely happy! THEIR's are genuine smiles!!
pretending in general give pain