It takes a while for Stefan's memory to come back, but when it does he realizes something is off. It is then that he suddenly remembers what Silas revealed about Bonnie before shoving him into that box.
Bonnie is dead.
Bonnie is dead and he soon realizes that the others don't even realize that she's gone.
He remembers them mentioning her on occasion (Caroline briefly mentioned she was out of town, he doesn't think Elena mentioned her at all). This had been when he still had amnesia and at the he assumed that she was a friend that they were no longer close to.
Now that his memory is back, he knows it's because they always tend to forget about her whenever she's not around. As a result, no one thought anything was odd about Bonnie not being around for several months.
And that is when he goes ballistic. He'd thought he'd been angry that it had taken them so long to figure out that he had been in trouble, but that anger has nothing on the anger his feels on Bonnie's behalf. So he decides to call a meeting to confront them all.
He asks why the hell no one found it strange that Bonnie would go through so much trouble to save Jeremy and then disappear for several months. Wouldn't she want to spend as much time as possible with the guy she brought back to life.
He asks them why they never questioned her not returning to Mystic Falls after her father was killed. Considering how loyal Bonnie is, do they really think she wouldn't come back to mourn her father's loss.
He wants to know how they never wondered why Bonnie would out of the blue decide to spend so much time with the mother that's abandoned her twice. Stefan thinks the fact that Abby hasn't come to see Bonnie once these last few months says a lot about the actual state of their relationship.
He rails at them because they should have realized something went wrong when it was revealed that Silas was back. Since they all rely on her so much, why would no one think to ask for her help. It's not like they have any trouble asking any other time.
They make excuses about receiving text messages and he calls them out on it. Because texts aren't the same as physically talking to her on the phone. A text isn't the same as hearing her voice or even seeing her face. And let's be honest, with all the technology that's out there, being able to see her face would be simple enough.
When he figures out that Jeremy had known this entire time, but never said anything, it takes everything in him not to strangle the guy because he definitely should have said something to others about this.
Stefan doesn't care that Bonnie asked him not to, that she believed nothing could be done. In Stefan's eyes the moment he got back, he should have told everyone what happened so they could have all found a way to save her. She went out of her way not once, but twice to bring him back to life. And both times she ended up suffering the consequences. So it would only be right for him to do the same for Bonnie. The girl he's supposedly in love with.
When all is said and done, he finds that he's kind of angry at Bonnie as well. Because she shouldn't have kept something like that a secret. Because not upsetting her "friends" should no longer be a priority for her. Especially when those friends don't even seem bothered by her absence. Why does she seem to think so little about herself? Why wouldn't she want them to try and save her the way she always saves them?
Suddenly it hits him that maybe she believes they wouldn't go out of their way to save her. That they would all forget about her and move on with their lives. And that happening would prove they don't really care about her, that they only cared about what she could do for them. That she basically sacrificed her life for people that wouldn't do the same.
He imagines it would be easier to deal with no one caring when they think you're alive than possibly finding out that the lack of caring also extends to your death as well.
And that is why Stefan vows to find a way bring her back, even if the others don't. Because she more than anyone deserves to have someone work night and day to bring her back. So she knows that someone does care.