Why blood becomes thick? ?today i visit for check up when dr taking my blood out with injection blood came very slowly and it's look very thick....

More likely: That the nature of the phlebotomy was suboptimal. If the needle is up against the wall of the vein, or just in a small vein, the flow may be slow and the blood may appear "thick". There are times where blood can get "thick" due to too many red blood cells, white blood cells or too much protein ... remember those conditions are quite rare, particularly in a 25 year old man.