Hailee gushed about how happy she is being in love with the former 'One Driection' band member.
She told Cosmopolitan in their December issue: "When you fall in love, you pick up on the tiniest things about yourself.
"I am one to spread (out) on the mattress and take up every inch of space. Now I don’t want to do that anymore. I want to make as much room for you as possible."
The 'Pitch Perfect' star added: "You start doing things for this person and going out of your way to make them happy.
"And when you see them happy, it makes you happier. When you love who you are with that person...there’s nothing that compares."
While the pair have never officially confirmed they are dating, they have been linked since November 2017 after being spotted together at a Backstreet Boys concert in Las Vegas.
Shortly after, Hailee posted an Instagram photo of herself in a T-shirt advertising Niall’s tour.
On his part, Horan posted a picture and wished Hailee happy birthday for her 21st last December.
Since then, the young'uns have been caught cuddling, smooching and holidaying together, taking a trip to Disneyland in August.