Fred has been in love with Angelina since he was fifteen. If he was ever called upon to produce a Patronus, he knows his happy memory would be asking her to the Yule Ball.

He'd been so nervous. He'd put off asking her for as long as possible, until long after George had asked Alicia and she'd agreed. He was finally pushed into action by Ron asking who he was going with. He could hardly tell his little brother he didn't have a date, so he said "Angelina" without even thinking about it.

He's just glad she said yes. How embarrassing would it have been otherwise?