Dawn couldn't help herself. She fell for you, her babysitter. Hard.
You were a close friend of Buffy, since as long as she came to Sunnydale, and at the same of Dawn too. You knew about her true nature but didn't care. She wasn't the Key for you. Just Dawn, Dawny, Dumby, and go on. And that maybe why she loved you.
Over time, because of the differents menaces in her life, and her younger age, you've took the role of babysitter too. Even if she was older now, you stayed home until Joyce or Buffy came back, cooked for her, watched cartoons with her and, sometimes, trained her to help her make a better use of her powers and physical abillities.
Today, wasn't an exception.
She saw you coming from her room's window (No! Of course she wasn't waiting for you!) and ran to the porch, to open the door for you.
And Christ...
You just came back from basketball training, sweaty, hair sticking on your forehead, sport bag slung over your shoulder, and a basketball in the crook of your arm.
It took her two minutes to understand she was staring openly at you. She crossed her arms over her chest with a huff before schooling back onto her usual sardonic expression.
"Go shower. You're going to make the house dirty."
God...
She was so whipped for you...