Cullen never feared magic until the "incident" at the Circle in Ferelden. He used to be cautious, suspicious, but he had been raised that way (and he honestly can't think of anyone who doesn't think that way, either currently or previously).

Finding out Lavellan was a mage unsettled him.
Having been raised by the Templars and going through what he did, followed by the destruction of the Chantry in Kirkwall by a mage, it's understandable to everyone why he reacts the way he does towards mages.

But Lavellan, it turns out, is so much more than a mage. She's a free spirit, used to travelling plains and forests; but still takes her responsibilities seriously.
Cullen doesn't know much about the Dalish, but he can understand that being the Keeper's First is both an honour and a burden.

Josephine told him what Lavellan told her; that she enjoyed roaming and exploring the forests they passed through, but that she was always a solitary child, even before the burden and responsibility of being taught by the Keeper was places on her frail shoulders.

Varric says she reminds him of Merrill in that way, and Cullen struggles to remember the petite elf who followed Hawke around.

Cullen know Lavellan isn't frail, but she looks like a gust of wind might blow her over.
He learns to see her as the tall grass after watching her fight.
The way she moved and bends and used her staff to cast spells; it seems like a dance she has practised many times over; either alone or in the company of her Keeper.

If Cullen were to describe her, he would say she were a a creature of the wild; something more than what everyone else sees; something more than a mage and more than an elf; as if nature had created her as a part of itself.

It pains him that there are no woods around Skyhold.
When they were still in Haven, he used to watch her wander among the trees, looking as she was communicating with them and the creatures in it.

Lavellan has a way with animals, and the look on her face when she found out that she could purchase a Hart for a mount is one Cullen would do anything for to keep on her face.
And when the mount arrived, she spent hours getting to know it, getting it to trust her and no one were heartless enough to tell her that she ought to go back to her duties instead.

It probably helped that he stood not far from the stables, glaring at the few people (Leliana and Cassandra) who tried to drag her into the War Room.
He had to endure snide comments about that later on, but it was for her and she deserves everything she wants.

(Even if it's someone else and he would have to watch her fall in love with someone who's not him. He would hate that but accept it, because he cares too much to loose her.)