Why is it hard for people to concentrate on everyday work when they like someone? or their crush touches them and that's all they can think of all day (just an example)

You're in the breakroom at work when you see a coworker posting a cartoon on the bulletin board. you read it as you pass, and it's a joke about women breastfeeding in public. a male walks up, looks at it and starts laughing. other make employees walk up, and one takes it down saying he is going to make copies for everyone. what do you do?