Ellie is not a serious relationship kind of girl. She's perfectly happy with her friends, her parties, and her meaningless hook-ups. In her experience, anything more than that leads to trouble. All she has to do is look at her mother to be sure of that.
Fred has made a career out of worrying. He worries about his parents, his burn-out sister, his screw-up best friend. When he starts hooking up with Ellie, he's ready for nothing more than some no-pressure, worry-free fun. Which is just the way Ellie likes it. So long as they stick to the plan, both can get out unscathed. So long as they stick to the plan, both can get what they want.
But life has a way of messing with the best of plans