Romantic comedy is one of the most contrived of all genres. It's literally a complex mating dance with prescribed story beats designed to allow the audience to feel the love the characters share. Which is why it is essential that you execute the form well enough so you don't let the contrivance, the mechanics, show.
Another word for story beats is plot. And lack of plot is the biggest problem writers of love stories have. Plot is what creates the magic in a story. It's the slight of hand, and mind, that delights the audience. It's also the structure that everything else hangs on. So when it is missing or obvious, especially in a romantic comedy, the story collapses and the audience realizes the magic is fake.
Dan in Real Life is the story of an advice columnist who falls in love with his brother's girlfriend at a family get-together. But this is not real life. Dan's parents live in a rustic little mansion by the shore. And his extended family has apparently won the Happiest Family on Earth Award. These people love each other so much that they spend their entire vacation in one uproarious communal activity after another.
The audience may well wish they lived in a family like this. But it is so far removed from reality that it becomes mechanical. The love between the characters is obviously being manufactured by the actors, because it has never been earned in the writing. And that makes the supposed love between the two leads seem manufactured as well.
But the biggest problem with this sequence of communal love scenes is that it kills the plot. The big reveal - that Dan has fallen for his brother's girlfriend - is in the opening set up. The rest of the movie repeats the same beat of yet another family get-together where everyone is having incredible fun but Dan. On those few occasions when the entire family isn't having fun, they are all gathered around in a kind of intervention/group therapy session helping Dan get his emotions and morals right.
