Why vein shrink and its treatment?

They don't normally: ...unless they are lasered or otherwise treated intentionally, or for example get damaged as a side effect of chemotherapy. I suspect you have had some difficulty with a blood draw? So, sometimes veins roll away from the needle. Sometimes they go into spasm and blood won't come out of them easily, and sometimes the technician is just not lucky! Usually they would just apply heat to the arm.