His job was to protect her.
Feelings weren't supposed to get involved.
Eve

I've been in love with my brother's best friend for as long as I can remember. Once upon a time he was my knight in shining armor, but now? He acts like I'm his worst enemy.

I don't know why Cooper Jenkins insists on keeping me at arm's length?which is hard to do when he's my partner at the Academy?or why he insists on shielding me from every other boy at school. I don't want to be at Oakwood high anymore than he seems to want me there, so why won't he leave me alone?

Cooper

When my best friend enlisted I made one promise: protect his little sister. And I'll do that, even if it means pissing her off and making her hate me. Besides, I'd rather she hate me than know how I truly feel.
If I give into temptation and make her mine, I'll be breaking my vow in every way possible.
But there's something about her.
Something in her gaze that is drawing me in and terrifying me at the same time. And the more I'm around her, the more I begin to fear that perhaps, the only person who can truly hurt Eve? Is me.

If you love big, teddy bear heroes who only want to protect the heroine and snarky heroines, you'll love the Wide Receiver and his Best Friend's Little Sister.