It’s really simple: BoJack wants to be appreciated, and has such deep insecurities that he isn’t sure that anyone appreciates him. When he receives this affection from a baby seahorse, he cherishes it, and the real BoJack comes out. The BoJack who saved Todd from Copernicus’ improv comedy ship is the same BoJack who is too proud to admit that he was paying attention to Todd’s rock opera. Those insecurities keep him from being good to people, as he always feels he has to cover his own ass, above all else.
He saves the seahorse because that’s the real BoJack. When he doesn’t have to worry about whether people will like him or not, he’s free to be good to people.
Now that (notice: this next comment may contain portions of the plot that occur towards the end of the season, just in case you have not watched the whole season) Sarah Lynn is dead, his insecurities won’t manifest themselves that way anymore. He has always seen himself as a father figure, whether it’s to Todd, Sarah Lynn, or as an actor in Horsin’ Around. He failed. The seahorse family is a trigger for that. It’s not that he doesn’t know what he wants. He just doesn’t think he deserves it.
The death of Sarah Lynn will force him to consider the feelings of others before himself. He genuinely cared about Sarah Lynn. Although they enabled each other as drug users, they were the only people who cared about each other unconditionally. He checked in on her dressing room in 2007 because of that bond, however messed up it was.
Of course, he genuinely cares that he was partly responsible for screwing up Sarah Lynn. That’s why he nearly committed suicide on the highway. (As Wendy Rhoades says in Billions, “Most people don’t manifest their guilt with this kind of behavior. Sociopaths don’t give a shit. You’re somewhere in the middle.”) I have a feeling that this will compel him to change for real. With that in mind, it’s utterly ridiculous that I have to wait another 12 months for the best show (barring the miraculous return of Review for Season 3, which was supposed to happen) on the web or actual television.