>>527641008
It all comes back to MoP.
Wrathion on screen is the only character Anduins "age" he's ever interacted with for a long period of time to this day and that was especially true back during MoP, when they had an expansion long storyline together hanging out with (you)
This is where most of the fangirls got the idea to ship them together as neither one a the time really had any on screen friends aside from each other.