Teen heartthrob Louis Tomlinson** celebrated his first anniversary with girlfriend Eleanor Calder Nov. 17 -- and he has One Direction bandmate Harry Styles** to thank for setting them up.

"Harry knew they would hit it off -- she's totally Louis' type," a source recently told Us Weekly.

Even so, it's not always easy for the University of Manchester student to date a guy chased by screaming fans. "They've had tough moments," the source explained, "but Louis is a hardworking guy; he would never give up when things get difficult. He's also extremely loyal."

A pal added, "They'll be together forever. You get that feeling being around them."

Earlier this year, Tomlinson assured Us he'd never break the 20-year-old brunette's heart. "I'm a one-woman man with Eleanor," he said. "I can be a romantic -- but not too sickly."

In a November Seventeen cover story, the "Little Things" singer, 20, explained why Calder is The One.

"Aside from the fact that she's hot, she had everything that I was looking for. She's funny, loud and confident," he said. "I'm not the type of guy who enjoys being single. I'm happy with the way things are with Eleanor. I told her I loved her after five months. I didn't just want up one day and think I was in love -- I knew it in my heart."

This article originally appeared on Usmagazine.com: Why One Direction's Louis Tomlinson, Eleanor Caldor Will "Be Together Forever"