Summer lovin', happened so fast.
"It had just been a casual hookup for Zac and Michelle. They were having fun," a source close to the situation exclusively tells E! News. "She is such a great person and he naturally gravitated towards that. They saw comfort in each other and connected on their experiences in rehab and were really good for each other.
"They were there for each other."
But apparently things got a little crowded on that yacht in Ibiza. (A yacht called the Ecstasea, no less.)
"Cara was always being mentioned," the source adds, referring, of course, to Vogue cover girl Cara Delevingne, who is super-close with Michelle. "Cara would spend time on the same boat and Cara and Michelle have all the same friends. It was just a lot. It was always, 'Cara, Cara, Cara.'"
And no one likes being a third wheel.
Meanwhile, Michelle is back in New York, as is Cara, and the vacation buddies were just hanging out yesterday at the Mercer Hotel.
"They are clearly best friends and they have never stopped talking to each other," our source says. "Michelle is very mature. She wants everyone to be happy."
Sure enough, Zac seems to be doing just fine. The Neighbors star has been spotted on multiple occasions recently working out in L.A. and he—along with half of Hollywood—took the Ice Bucket Challenge, literally seconds after pal Ashley Tisdale nominated him.