That part is the only complaint I have when it comes to the series (which I love, love, love). The books did a much better job at describing Holden's motives. Making book-Eros come alive on screen would have undoubtedly cost a lot of money, money I guess the showrunners did not have, but TV-Eros and book-Eros were not on the same scale. In the books Eros was truly terrifing, you understood without other characters having to tell you that Holden's and Miller's life changed forever after they escaped from the tunnels of hell. It was chaotic, horrendous, a run for their lives and it was chrystal clear that everything Jim did after Eros was because of Eros. In the show it was some empty corridors, with couple of bodies having brown gooey coming out of them, and a radiation sickness which was cured with band-aids. So it is a bit harder to understand the gravity of what Jim and Miller experienced down there.
Also, the books did a better job with putting one life-altering event after the other and also with the connection between Holden and Miller: the destruction of the Canterbury, the destruction of the Donnager, findig Julie on Eros, the massacre on Eros, the killing of Dresden, the death of Miller came in a short period of time, and while Jim held himself together up until the last event, the death of Miller shoved him over the edge. By the end of the first book you were just as terrified of the protomolecule as Jim was.