What were the characteristics of a good girlfriend?
Everything that wasn't her.
Tammy wasn't good with mush, couldn't stand watching romantic movies without wanting to hurl. She didn't have songs with her significant others. Hell, she didn't have significant others.
So when Fletcher needed a fake girlfriend to take to a wedding—one who wouldn't get attached like all the other women who fell in love with him at the drop of a hat—she was the obvious choice.
She went, and everything was great.
Until Fletcher fell for her.
Until...she fell for him right back.