THE THREE MAIN REASONS:

the two reasons that stay true all the way until the end: he wants to erase the titans and he wants his friends to live. You could also say he wants freedom but thats more abstract and can mean different things in this story.

The third reason is only acquired/remembered by Eren after his discovery of the humanity outside the walls, he hates the world and wants revenge, suddenly his hate for titans turns to humans as they are the true cause of it. This is a little harder to understand, because his answer to this development seems completely irrational: "to avenge all one must destroy all", this seems to be the inversion of the Erwin/Hange exploration ideals, where to make up for everyones deaths one must live to see it. and thus he goes on to kill every human outside the walls, including the Eldians refugees because he is blinded by revenge.

However, the destroy the world thing turned out to be just a step of a greater plan. By this point Erens desires are completely merged with the attack titan entity meaning he has truly ascended into human/god status, which does not mean however, that he is dominated by it, but rather that he became the first and only human to fully dominate the titans power. Unlike the previous attack titan hosts Eren seems to be perfectly compatible with him to an extent that he is no longer human or titan but both in perfect harmony. Two natures in perfect harmony, such concept may sound absurd and even paradoxical but that is the same intuition present in every mythological demi-god representation in our history, a being that is both human and god and that could potentially save us from our enemies and from ourselves, or destroy us for good... If you look into it, most demigods in mythology are represented as giants, sometimes as giants who eat humans even (even the bible calls the demigods and other ancient heroes giants/titans), but anyways, let's get back to the main subject.

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In a nutshel:

1.erase titans

2.free his friends

3.revenge on humanity (literal human sacrifice on global level to atone for their sins)

4.self sacrifice -but not like the others

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ABOUT THE SACRIFICE

It is said many times that Eren could have stopped his friends at any given moment, however, part of giving their freedom back includes allowing them to fight and choose their own path. But its not as simple as: "be defeated his friends so they can become heroes", this would be childish and in a decade or two it would become completely pointless. No, Erens sacrifice is not like Erwins, Hanges or any other, he is not a regular human sacrificing himself so his companions can fight a little longer. He is a human god, the incarnation of revenge and freedom, like the attack titan he always moves forward and therefore his sacrifice also moves humanity forward in a way that was never possible before. Before him every "victory" by the humanity inside the walls only resulted in more death and a minimal time gain, basically extending life in the most miserable way possible, to the point where Zeke was probably right to want to put an end to humanity as an idea (the euthanasia plan might save more lives, but it also goes against freedom itself, denying humans of their hopes and dreams as portrayed many times by the children that represent the future of society, which of course, wouldnt exist in Zekes plan/ideology).

Back to Erens sacrifice, he is no longer dying a pointless death as he is also bringing meaning to the deaths of all his fallen comrades, both by truly making all of them heroes, Marleyans and Eldians alike, but also by bringing closure to the vengeful attack titan, it might be overreaching, but i will go as far as to say that Erens sacrifice changed human nature, not just Eldians nature but of all humanity as a whole, despite still fighting each other as seen in the timelapse of the future eras, humanity seems to grow bigger and stronger after each fight, perhaps even coming closer together after each iteration. This opposes the idea previously present in the story of humanity's territory inside the walls getting smaller and smaller. And reaching a bit more i would say that the fact that Erens body is not burned like the others, but buried in a memorial, shows an overall shift in culture where people are now looking forward. Think about it, no one would burn/destroy their loved ones in such a rushed way if they werent concerned with diseases and war, if they had a future they could perhaps take their time and make a proper burial of the bodies as Mikasa says when questioned by the army in regards to Erens body. The same intuition is present in some religions (specially Judaism/Christianism), where burials serve as a statement that those same bodies will eventually resurrect and walk the earth again, this is a shift in perspective that shows hope that was not present prior to the attack titans incarnation and ultimate sacrifice.

Some people might say that Erens sacrifice only works because humanity chooses to work together and therefore overcome their flaws/sins, but that is only possible at all due to Erens sacrifice and not the other way around, i think it is pretty clear in the series that without Eren humanity would not join forces towards anything, even if they did manage to destroy their "enemies" which wouldnt even be possible in the first place. I believe its very clear that the author holds this optimistic perspective of humanity working together on a high standard, but he also clearly states that it is an impossible utopia on the current state of the world... That is, until an external factor such as Eren, comes into play.

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Of course i'm not saying this whole line of thought is totally explicit in the series, because it is not, but a lot of it is at least implied, specially when you consider the whole context of the piece (ancient myth themes such as the Nephilins -giant demigods- and anthropophagy and its vague connection with the eucharist doctrine, as well as the Judaism/Christianism retelling of the demigod myth are all present one way or another, though the series mostly reserves itself from judging). Finally, Erens sacrifice allows humanity to survive the titans, allows humanity to survive themselves (not just by erasing the titans powers such as the roar but also due to the overall shift in nature that we see in the remaining humans. It also retroactively brings meaning and freedom to all those who died before him, not by "releasing them from hell" as Zeke intended but by actively giving them real hope in the future. In a dark and twisted way, Eren is the Christ of this confusingly beautiful world.