Why do people think you are staring at them if you look at them for one second?

They proabbly have bad anxiety or feel nervous around people

cause they dont like to be looked at

some of them get mad

If it's obvious that your attention was on them, they'll assume that you were staring at them.  There are cultural differences at work – in some places nobody cares how long you look at them, but in others even a short "stare" offends them.  

Your own facial expression makes a difference, too.  If you show even small signs of disapproval, they'll be offended.  If you smile too broadly, they might think you're trying to "come on" to them.

Real people are not like people on a television screen.  Real people notice how long you stare at them.

Usually because these people get stared at by other people who are rude about it. I was once staring at a woman on the train but wasn't aware of it - she was! Before she got off - she abused me. The shame of it is, she didn't know I was staring because I was so jealous of her looking so good in the fashionable pants she was wearing. Something I could never do because of my thick thighs. It was only after her abuse that I realised she was black. Hand on heart, I didn't notice during my bout of envy. One point? To these people - sometimes people staring aren't doing it to be nasty. Second point? Like in my case - be a bit more sensitive before staring - better still - TELL the person why you're envious!

Some people can become quite concerned if someone looks at them, no matter the duration. While the intention may not be to alarm others, the recipient's response is valid for them.

cause you were looking at them