"We're completely smitten for each other," Jenner, 25, tells PEOPLE. "I'm loving life."
The couple, who've known each other (through one of Jenner's exes) for more than two years, had a bit of a false start when Jenner asked Nicole, 22, to meet him at a San Diego club, where he was hosting an event, and she didn't show up.
"I was totally heartbroken," he says. "I was very upset because I had a crush on her."
Now, they're a couple and Jenner attributes their success to knowing each other for so long. "The reason why I love this girl so much is because she knows me," he says. "She gets it ... She knows my background."
Traveled TogetherMaking time to travel, the pair have visited St. Thomas, Hawaii and the Bahamas. "She's a great travel partner," he says.
Back in L.A., the Bromance star says they're more likely to stay at home than hit the clubs. "We stay in, maybe bake cookies," he says. "It sounds kind of lame, I know – sorry, boys! – but we bake some cookies, watch DVDs and have nice quality home time."
"It's a change in my life, but it's a good change," Jenner says.
Inspired by his girlfriend – "She's in amazing, amazing shape," he boasts – Jenner says he feels "incredible," doesn't think about alcohol and doesn't want to drink again. "But I have to on Feb. 19," he says. "It's my lady's birthday."
What else does he have planned? "I want to do something very special for her," he says, after getting the perfect Christmas gift from her: a custom surfboard, emblazoned with a photo from her Playmate of the Year spread and the Playboy Bunny logo.
"She's naked but it's very artistic," Jenner says. "It's black and white. It looks incredible."