In Spider-Man 2, I understand that Doctor Octopus is being controlled by his tentacles, but what is his actual motivation for rebuilding the "device"? What does he hope to achieve by doing this?
Does he hope to destroy the city? Does he just want ultimate power? Is he trying to prove that he can actually control what he's created?
His motivation always seemed a little vague to me.
The arms appear to be only semi-sentient, largely acting in response to Octavius' subconscious desires; especially those regarding his desire for scientific success and his need to atone for the dismal failure of his earlier experiment.
Their key symptom is to utterly remove any semblance of impulse control from the Doc, driving him to undertake acts that he would never seriously contemplate (let alone act upon) if he was fully in control of his faculties.
"You worry about too many things, Father [said the tentacles]. That is why you cannot focus. You must start at the beginning. Make one thing right, and everything else will follow. The first thing you have to do is rebuild the fusion reactor." As much as he wanted to ignore the voices, Octavius felt the need to respond. Perhaps he could argue them into going away. “Rebuild? Why? It didn't work. Peter Parker was right. I miscalculated, and now they're laughing at me.” Do not be absurd, Father. You are the great Octavius. Everyone knows it.
There's a scene where the arms are clearly convincing him that he should build another one, and he isn't able to think clearly enough to realize why that is a bad idea. So I think it's more, "What's the arms' motivation?" My guess is that he built the arms in order to build the device, so when he created them, he gave them the compulsion that the device had to be completed. But I do not think it is ever outright explained in the movie.
I mean, it was his life's work. Plus, he and his wife worked together on it, so he might want to carry it on for her. I mean, there were all sorts of things going on and he was panicking and like others have said, probably had quite the lack of impulse control due to the inhibitor breaking. There are many reasons he might do it, but people tend to forget that one's acts are rarely rational after a tragedy. Especially one they caused. He might have just been looking for something to lean on, a way to "fix" things. Who knows though.
