Thanos is basically an interstellar tyrant who has been moving across the universe enacting his idea of balance by reducing the populations of existence by half wherever he goes. He, apparently, is trying to realize this ambition for all of creation. I guess he figured out there's no way he's going to accomplish this in his lifetime leap-frogging from galaxy to galaxy, so he seeks tokens of power called the "Infinity Stones". They're supposed to give a person absolute power over the given affinity. For example, the Time stone gives you near-absolute power over all aspects of time. Of course, in order to make a movie where the heroes of Earth matter, these Stones are not even close to that type of power because, technically, Thanos possessing just one of them should be enough to extinguish all life on Earth.
Thanos has collected the other 4 stones by force. The fifth stone, however, is special. To me, it's special for a number of reasons. One, that whatever is capable of resisting a person who wields 4 of the Infinity Stones- that means a person who has absolute control over every other aspect of existence except for souls- from taking it by force must be an unreal power in and of itself. But, two, it presents a pretty basic, but fascinating philosophical paradox. Upon approaching the Stones resting place, Thanos is told that the Soul Stone will not allow itself to be possessed unless the seeker sacrifices that which he loves the most. To meet this demand, Thanos chucks his daughter in for the deal.
And this blew my mind. Because I felt like they wasted such an awesome moment. Let me explain...
The paradox is that you have to give up something you love...to get something you desire.
Now...love is hard to describe. It comes in degrees. However, regardless of the degree, is there a level of love where you will let something you love die for the sake of something you desire? I mean...you might say yes, but then is that actually love? Also, this can't just be any love. What kind of price is that if you can just throw your poodle into a pit to gain absolute power? I'm thinking this must mean real love. This is my opinion, but while at the base level, love and desire are not synonymous, I think if you actually have that which you really love, then you already have what you desire and you would not seek anything else. In other words, you do not always love what you desire, but you will always desire that which you love. You certainty wouldn't give it away to get something that would be, in contrast, inferior.
I thought this scene was gonna get deep because Thanos was dragging his daughter behind him, convinced that she was the one he loved. He would sacrifice her, but then figure out the stone still rejected him. Why? Because he was only convincing himself that he loved her the same way he has convinced himself that he's actually doing a good thing for all of existence by killing half the people in it. But at this point Thanos would have realized one of two things... One, his actual love is power. Or two, his actual love is himself. Neither of these things can be sacrificed while still attaining the Stone, meaning that no person can actually hold the Stone.
This realization would have made more sense because the Avenger's are basically fighting God, otherwise. However, if Thanos was the one to destroy himself...
Thanos is still mortal, with a mortals mind. Having it broken by revealing the truth of his own psychological charades would have broken his resolve to continue his path. (I think a really cool requirement to gaining the soul stone would have been to make a person face those things they hold deep down in their souls. This might sound like a triffle, but there are things we would kill to keep from confronting, to keep hidden, etc. That darkness in the center of the heart or soul...that's hell. And all men tremble upon approach)Trying to gain absolute power over everything. That's an impossible thing that demands an impossible price. And that is the lesson the Stone would have been trying to impart. Absolute power is an illusion you cannot achieve. Attempt it, and you chasing your own tail, never achieving what you desire because by giving everything to attain everything, what have you gained in the end?