Why is it that when somebody is on your mind, everything around you reminds you of them?

Everything you read reminds you of something they said
Every song you hear resembles how you feel about them
Everybody you come across is somehow linked to this person
Every movie you see makes you want to call them and talk to them
Everything that happens reminds you of a conversation you had with them or something that you did together
Everything that you plan on doing, somehow revolves around having this person around

And no matter you do, they occupy every thought in your head.