Ugh, don't say such awful things, Ea!
Can you imagine how depressing it would be if Sam pushed Frodo in or pulled him in or what have you? But that's actually kind of intriguing, because I'm not sure if Sam actually would have done it. Think back to
"The Choices of Master Samwise"; Sam does take the Ring from (he thinks) Frodo's dead body, but the instant he hears orcs, he races back to try and protect
Frodo's dead body from being, I don't know, dishonoured. He chooses
Frodo's dead body over continuing on with the Ring. He chooses
Frodo's dead body over Middle-Earth, in a sense. It was more of a gut reaction than a conscious decision, but he still went back. He
knows that the Ring needs to be destroyed-- that was the whole point of the chapter, Sam trying to figure out what his duty is now that Frodo's dead, and he decides his duty is to finish the Quest, but when it really comes down to it, he doesn't care, because his loyalty to Frodo trumps all.
"Don't you leave him, Samwise Gamgee. And I don't mean to." I think Sam is so loyal it's almost twisted.
Maybe he's a Hufflepuff. 
Also: Frodo's gloom and doom mentality. I think it's rather tragic, of course; Frodo, who loved the Shire, and thought he wanted to go adventuring but then found out he was wrong and the Shire was his home after all and he wanted to go back but he couldn't, because this thing had gotten bigger than what he wanted, and he had to see it through. But I think Gandalf's death was the first blow and Amon Hen was the death stroke as far as holding onto any hope that he'd come out of this alive or even succeed in destroying the Ring. I can't really imagine what that would have been like, marching on into Mordor, completely convinced and resigned to the fact that sooner or later, you're going to be caught and killed.
The idea of going on with
no hope. I mean, we can chuckle a bit, (I know I do, sometimes) but the fact that he keeps going despite it all is, I think, a testament to his strength of will. It's generally against one's nature to continue on in a task that you know is going to fail and be for nothing.
Now, on to Sam! For Sam, in the beginning, it was very simple: Mr. Frodo was going into Mordor, and so that was where Sam was going. He didn't think too far ahead, other than to make vague plans for 'when it was all over'. He followed Mr. Frodo and he protected Mr. Frodo and that was that. But then, at the beginning of RotK, they switch roles, and now it's Sam leading and Frodo following in his footsteps. I think Sam's motivation was almost the same; Frodo had to destroy the Ring and he couldn't do it by himself so Sam had to help him. But Frodo is now following Sam's lead, and if Sam sat down or gave up, I think at that point Frodo would have as well.
*phew* Trying to understand hobbits is hard work. Nurr? Am I committing blasphemy?
Without him Frodo would have just quit the whole thing and found a quiet pit to die in. And while I think that Tolkien does go over the top with gloomy, depressive emo Frodo to the point where you just want to whack him (well, Frodo, not Tolkien) over the head.
I was just wondering about Sam.. because when he turned to go back to a presumably dead Frodo they were still on the border of Mordor - there was still hope and at least Sam might have had a memory of sunshine and mushrooms and a cosy fire. It would have been easier to act rational. Whereas, after they had walked all across Mordor, fled and hid for days, almost been discovered by the company of orcs, having no food, no shelter.. Even Sam's spirit must have been on crutches at this point. I don't know... However, I'm sure he would understand that they would have to walk in a dead and barren Mordor forever - figuratively speaking - if Frodo kept the Ring. Which wouldn't be long, being on the threshold of Barad-dûr they probably wouldn't have to wait long before Sauron would come and ask to get his Precious back. So the question is down to how much trust you put in Sam's loyalty to Frodo here and there. And you might be right, that his love and care for Frodo might win and stay until the end. After all, this is how Tolkien wrote his character. I just can't help thinking about Sam the Hero. He knew how hero'ing worked because he knew so many of the old stories and the awareness of being a part of something so much bigger might have influenced him too.