The thing that I can't make me understand is why my character would romance Cullen?
They have nothing in common, why my character will start to flirt with him?
Maybe just because he looks like the prince Charming
I thought the same thing the first time I played (since, as you can tell by my username and picture, I'm pretty invested in "elfy" stuff), but you'll find that his personality is very agreeable. He's largely tackled his issues from the previous two games, and by this game he's almost unfailingly kind, polite, honest, sweet, open-minded, helpful, considerate, etc. (He's almost too perfect.
)
There are also a few points in the game (depending on your choices) where you and he can fight side-by-side, and since you run the Inquisition but he leads your armies, at times it can feel like you two are equal partners working toward a common goal, rather than you're the player character and he's your underling.
The very first time I played, my pro-Dalish Elven Inquisitor initially thought, "Hiss! Templar! Get away from me!" (it helps that I headcanon that she was a city elf whose parents fled to the Dalish when she came into her magic, and they were chased and her father was killed by Templars on the way), but he slowly grew on her. It helps that, as an Elf, she doesn't care for human politics or power-grabbing, or how fake, insincere, and superficial Orlesian society is (Orlais is the Capitol from Hunger Games, and at times it felt like my gal was a District 12 newcomer), and Cullen expressing disgust for such things too, in favor of an honest day's work and a desire to help people for its own sake, appealed to her.
There are also some instances where he's good to elves specifically. If you wander around Haven, you can come across a human soldier apologizing to an elven scout, and he'll mention how Commander Cullen chewed him out for calling her... you know... and made him apologize. You'll also encounter a war table mission where your clan is attacked by bandits (it's the second "personal" war table mission, after "Contact Clan Lavellan"), and you can have Cullen send his soldiers to defend them. It's such a little thing, but the fact that Cullen, a human Templar, successfully defended your friends and family (Dalish elves) from being killed off, can feel like a big deal. I didn't expect it to happen, but my Elven Inquisitor's regard for him shot up 50% just from that alone.
There are also a few instances in the romance where you can bring up the "elf" issue, and his answers alone can melt hearts.
If my gal hadn't fallen for Solas first, she would have gone for Cullen just based on his good moral character, and general acceptance and open-mindedness this game.
In fact, the two wound up having so much chemistry that I actually did start a new playthrough with the same character, and romanced Cullen that time. 
So yeah. There's common ground if you're willing to find it. 