Do I smell a rat?
One key characteristic of red herrings, apart from their colour, was their strong smell, so much so that one use for them on occasion was to train hounds to follow a scent.
a deliberately misleading object
Red herrings were often used to mislead hound dogs; the expression refers to something that is misleading.
Therefore - you try to convince yourself that you like someone (to mislead you/lead you away from the person you actually like), and call him/her your 'red herring'.
B: I'm so sorry :( what are you going to do?
A: I think I might go for Tony.
B: Tony? Why! Do you even like him?!
A: Nah, but he'll just be my red herring.
The red comes from the color of it, and the herring is because it smells like fish.
I was with my girlfriend last night, but I had to leave, because I discovered her red herring.
President Bush: "Casualties, man gay people want to get married!"
Commentator: "Wow, did you see that red herring Bush just threw out there? Amazing stuff folks!"