Roberto Orci and Alex Kurtzman,in a MTV interview, explain the reason behind the decision that made us all happy fangirls :)
Now that we can safely assume that most of you are past spoiler-warning territory (if you haven’t seen “Trek” by now, um, why?), here’s a few additional tidbits we were holding back. They come courtesy of “Trek” writers/producers Roberto Orci and Alex Kurtzman, who recently told us about what they consider to have been their two gutsiest choices while writing the script.
[the first choice is destroying Vulcan...]
Discussing the reasoning behind the romantic subplot, Orci explained: “It was a decision that was made largely because Spock had just suffered the death of his planet, and you felt for him so dramatically, and in a moment like that the person he would talk to - in his own Spock way - would have been Kirk, except him and Kirk weren’t friends at that point.”
“So, it felt like there was an organic way to make Uhura his secret confidante,” Orci continued. “What happens in that scene is that you are paying tribute to a variant of what happened in the original series, and want Spock to have a hug in that scene. And because he can’t express emotion - well, he can but he chooses not to - we felt like [Uhura kissing Spock was] a really good way to allow the audience to do that for him.”
source ETA: I forgot this part, from another source sorry :)
A lot of people found themselves scratching their heads over the unlikely romantic pairing of Spock and Uhura. The inspiration for this came from the original series, where apparently there are scenes of these two flirting [...] Since the rough-and-tumble badboy is always the one to get the girl, the writers wanted to pair Uhura up with the less obvious choice. Besides, since Uhura is a smart, mature woman, they felt that she would probably gravitate towards the more interesting, intellectually mature man.
source