Why do customers get upset when they are asked for their ID?

I am a bank teller and on a daily basis I get customers huffing and puffing because I ask for their drivers license when they are cashing a check. I get to know the customers...but if you only come into the bank once a month, chances are that I don't know you. Just because you have had an account for 30 years doesn't mean you do not have to follow policy of requesting ID on cash transactions. We are allowed to refuse service when the customer doesn't comply with this policy. What is the big deal? I would like to hear from a person who would get upset with me if you walked into my bank and I asked for you ID when you are cashing a check or getting cash back on a deposit.