Why thawed food should never be refrozen?

Thawed food should never be refrozen, because once it is thawed, pathogen growth may have started. 

Pathogen growth takes place very slowly at refrigerator temperatures, but it does take place, and some pathogens are destroyed by freezing, and some are not which may produce toxins.

Along with this, water expands when it freezes, so the water inside cells freezes, expands and breaks the cell membranes, which makes the foods pulpy and leads to change in the quality, taste, and texture of the food.