"I just thought maybe we could write this [song] together."
In season 1, Ricky can't/isn't ready to express how he feels about Nini in his own words, so he uses her's instead (by singing her song, "I Think I Kinda Ya Know", back to her during auditions). It's not very well-received, at least not by Nini (Gina's eyes light up when she hears Ricky sing for the first time). Partially because Nini has a new boyfriend and she doesn't appreciate Ricky effectively forcing his way back into her life by trying out for the musical...but mostly because he still can't say "I love you" to her, even if it's in a song (that he didn't even write). Later, Ricky wrote "Just for a Moment" for Nini, but never actually sang it to her (at least not in their reality...and when he did sing an excerpt of it to Big Red, Red immediately thought it was about Gina).
Then, in season 2, he writes Nini yet another song (two, actually, but "The Perfect Gift" is so generic that I consider it more of a cute holiday song by Josh than a song Ricky wrote for Nini) — and again, he doesn't actually get to sing it to her. They're so bad at communicating with one another that they spend the entirety of that day (Valentine's Day) trying to meet up, but never being able to. They can't even get the timing of a simple phone call right (a call that was Gina's idea, by the way) so Ricky has to sing "Even When" to Nini's voicemail (and vice versa).
Later on in season 2, Ricky & Nini were tasked (by Miss Jenn) with writing an original song for Beauty & The Beast together and they couldn't do it. Nini, instead, wrote the song on her own — a song that ended up being about how she feels, understandably, idolized and suffocated by her relationship with Ricky...which she told Kourtney not to tell Ricky about and then told Ricky, to his face, that the song wasn't about him.
Ricky writes Nini another song ("Let You Go") after they break up (or at least he finishes it post-breakup) that he claims was written for Nini, as a goodbye or some kind of "Hail Mary", but he never sings it to her...because it wasn't so much about her as it was about him grieving the childhood relationship they had and coming to terms with the fact that ending it was the right choice for both of them.
My point being, Ricky has always written songs for someone else (or claimed they were for someone else), but never with them (technically, there's that throw away line in s2 where Carlos mentions that Ricky wrote some of the lyrics in "In A Heartbeat", the song for Seb, but I'm talking romantic relationships here). He couldn't write a song with Nini because they were never on the same page, they couldn't communicate with or understand each other personally, so they couldn't collaborate musically (& even the songs he wrote for her rarely went the way he wanted them to).
Which is why it speaks volumes when Ricky says he wants to write a song with Gina. Finally, he's in a relationship with someone that's built on true friendship, mutual support and understanding (& vice versa for Gina, too), someone that he can communicate and collaborate with. With Nini, it was always about pushing his feelings onto her, but now he doesn't just want to sing a song to Gina about just his feelings (though he'll do that, too) he wants to listen to her, to know how she feels. He wants to work together to create something that they can share, that's not just his or just her's, but uniquely their's.
In season 2, when Nini flees after Ricky tries to talk to her about the song she wrote ("The Rose Song") she says something like, "It's just a song, Ricky." to which he responds, "A song can mean everything."
He was right, a song can mean everything.
