Love is NOT all you need.
Love matters, of course, and it's an essential part of raising a child. But millions of parents who love their children still end up screwing them up so they end up in therapy. Love alone just doesn't cut it. Really, when a film takes on an issue like this, it has to fairly present both sides of the argument, not portray Sam as an angel and the big, bad legal system as a bunch of over-the-top heartless demons.
The subplot with Rita and her strained relationship with her son just felt like the film was hammering me over the head with, "SEE?! Rita is normal but still a bad mom, therefore Sam should get Lucy!" Wow, that's a stretch. And I bet you Rita loved her son as well, but that didn't make her a good mom, nor does love make Sam a capable parent.
To those who say taking care of Sam will make Lucy smarter, more mature and compassionate... on the contrary, she could turn out to have co-dependecy issues, be unable to relate to children her age, and may resent her father someday. Maybe in her teen years she'll get fed up looking after him, crave some independence and enter a rebellious phase, which Sam won't be able to handle.
Granted, the movie ended with a good compromise: Sam and the adoptive parents raise Lucy together. That's great. But the rest of the film is way too one-sided and simplistic to be taken seriously.