A theme that resonates with your audience can excuse a multitude of contrivances.
I was mostly as happy with Crazy, Stupid, Love as the critics and opening weekend audiences who gave this modestly ambitious romantic comedy a good buzz. CSL delivers an appealing cast, some sharp, savvy dialogue, a couple of solid wish-fulfillment fantasy moments, and enough honest laughs - i.e. the kind that come from truth-telling, as opposed to mere outrageous absurdity - to win over all but the most hardened rom-com-averse moviegoer.
For the record, the movie succeeds in mastering a particularly challenging sub-genre: the ensemble comedy. Only a handful of respectable rom-coms have pulled this off in recent memory (most notably, that saccherine stealth-bomber Love, Actually, and most execrably, that wholly bogus drek-burger of a wallet-stealer, Valentine's Day).
CSL's screenwriter Dan Fogelman has constructed a Chekhovian roundelay of interlocking story-lines: Cal (Steve Carrell) loves unfaithful wife Emily (Julianne Moore), while their son Robbie (Jonah Bobo) loves babysitter Jessica (Annaleigh Tipton)... who's got a mad crush on his dad, Cal. And that's not all, though to reveal exactly how co-stars Ryan Gosling, Emma Stone, Marisa Tomei, and Kevin Bacon fit into the larger scheme would be a Spoiler Fest (meanwhile, the movie's a bonanza for Six Degrees Of Bacon charters).
Without giving the game away, I'll note that in the time-honored tradition of planting and paying-off, Fogelman has played his hand perfectly. Via one deft omission early on, the movie suprises us with a funny, unexpected reveal at the midpoint: someone we didn't expect to see shows up in an unfamiliar context, and high jinks ensue. Having accomplished this, the screenwriter subliminally sets us up for a far more significant surprise in the climax - we're willing to accept what might otherwise have seemed a credibility-busting, out-of-left-field development, precisely because we've been successfully misdirected (and enjoyed it) once before.
Well done, sir, but even so: Given how bizarrely coincidental the story's entire construct actually is, it's kind of astonishing that we can suspend disbelief and buy into it, and I found myself wondering why CSL's plot worked so well, where that of so many other ensemble rom-coms I've read or seen have not. The answer lies not in our stars, but in the theme.
The glue that binds ensemble stories is thematic; though movies with ensemble story lines tend to be anchored around one prominent POV protagonist (e.g. Carrell's Cal here, and the Steve Martin character in Parenthood), theme is what holds the disparate subplots together. Crazy, Stupid, Love has not one, but two predominate thematic threads to do the job.
One is articulated regularly on-screen: the idea that soul-mates exist, and that you have to fight to win them if you believe this (Trope sound familar? See this post). The other is never directly spoken of by its characters, but I'll put money on it as the foundation of Fogelman's grand scheme: the idea of forgiveness.
Again, to avoid spoilage, you'll have to take me on faith unless you've already seen the picture, in which case, think about it: What is the true connective tissue in CSL's construct? On close examination, in each and every story line, you'll find a character who has hurt or betrayed another, or behaved reprehensibly in some significant way. And the means by which a happy resolution is achieved for that character is forgiveness.
Forgiveness as a positive, transformative force is the fuel that powers what would otherwise have seemed a pile of ridiculous contrivances in Crazy, Stupid, Love. While its soul-mate theme appeals to the idealist and romantic in us (as the sentimental "Love is all around you and you only have to recognize it" idea more or less makes Love, Actually fly), the "positive value of forgiveness" theme addresses our moral core.
It's hard to forgive people, often hard to forgive ourselves, and this stubborn truth about being human grounds the farcical shenanigans in CSL. All of its characters struggle with this issue - it's actually what the story is about - and all but one manage to forgive in the end (the one who doesn't, tellingly, remains a comedic write-off). We accept the absurdities of the plot because we want to believe in forgiveness, ourselves. On a thematic wish-fulfillment fantasy level, that's why we leave the movie feeling good.
You can pick and choose among the Crazy, Stupid, Love stories as to which ones you love and which you deem merely crazy or stupid (I liked the Ryan Gosling-Emma Stone stuff the best, while finding the adolescents' story borderline horrible), but rom-com screenwriters ought to check this movie out for its use of canny theme-wielding as a license for contrivance. That's my theme and I'm sticking to it.