Ian Kershaw's biography of Hitler sheds some light on this.

Hitler personally viewed surrender as cowardice, and believed that the German Army in WWI had been betrayed by politicians at home, despite seeing success in the trenches. He blamed not only Jews, but Marxists in the German Imperial government.

His sense of betrayal was formative toward his political ideology and would lead his state into an ideological struggle against the USSR. By 1945, from his position, death was preferable to surrendering to the forces of an ideology he veiwed as cowardly, backwards, and responsible for the prior humiliation of Germany.

Germany's involvement in WWII was that of instigator, as they sized territory and eventually invaded Poland. In WWI they were acting as ally to the Austro-Hungarian Empire, who were pressing the Serbs in response to the assassination of Franz Ferdinand. Hew Strachen writes in his book "The First World War" that the Germans saw opportunity, being more prepared for war than France or Russia at the onset, and pledged support to the A-H Empire, with the assumption that Germany would press west, and the Austro-Hungarians would press east. The lackluster planning and communication errors were costly, and Germany found themselves fighting a two front war, which they wanted out of by the end of 1916, as evidenced by their seeking terms with allied powers on 12, Dec.

The short answer is that in 1945 nobody wanted a negotiated settlement. The long answer is a bit longer.

Hitler had always seen the conflict as a 'war of extermination' in which there could only be one winner. (Which led Stalin to retort that "If the Germans want a war of extermination, well, they will get it!") This was not 'just' a war but a racial struggle for the very survival of the German Volk. And this was reflected in the increasing radicalisation of the Nazi regime during the war; – the countless massacres (particularly in the East), the Holocaust, death marches, etc – all ensured that by 1945 Germany's bridges were well and truly burnt. The war could only end with the utter defeat of one party; indeed the Allies adopted such a stance (ie an insistence on unconditional surrender) at the 1943 Casablanca Conference. Germany was simply in too deep.

But that's only part of the equation. In 1918 powerful forces within Germany had conspired for peace. The economic collapse of 1918, coupled with the danger of a communist revolution, convinced the Imperial German Army (particularly Ludendorff) to sue for a negotiated peace in an attempt (successfully, as it happens) to preserve the Army as an institution. Once the Army decided to that peace was necessary for its own self-preservation then Kaiser Wilhelm II had little choice in the matter. (Of course the Army then cynically handed over to a civilian government and then criticised them for 'stabbing the army in the back'.)

That is, having being brought to the brink of complete national collapse, there were institutions in Germany in 1918 that were capable of recognising this disaster and pressing for a negotiated peace. In 1945 such institutions did not exist. The Wehrmacht was far too tainted by war crimes (and too in thrall to Hitler) to offer a meaningful alternative and political opposition to the Nazis (ie the SPD or KPD) had long since been eradicated. So even if the Allies had been minded to negotiate a settlement, there was nobody they could realistically talk to.

(And, off the top of my head, no. Germany in 1918 surrendered without any Allied soldiers on its soil and with its army still in good order. That does however underplay the degree to which the economy was collapsing and discontent rife. Indeed, the German Revolution had begun with a naval mutiny at Kiel on 03 Nov 1918.)

Sources: Kershaw's Hitler has already been mentioned as the definitive work on Hitler but almost all books on the Nazi Reich (I'm thinking Hitler's Empire and Wages of Destruction) will touch on the radicalisation of the regime. That famous Stalin quote comes from a speech on 06 Nov 1941 (https://www.marxists.org/reference/archive/stalin/works/1941/11/06.htm). On the end of WWI and the Army's role in this, I can't recall if I got most of the details from Detlev Peukert's or Eberhard Kolb's The Weimar Republic. Peukert's work is excellent though and contains a great analysis of the degree to which the Army (and other Imperial institutions) were able to first engineer and then blame the new republic for Versailles.

As I understand it, Hitler was totally insane in those final days and making all kinds of ridiculous promises about new weapons, a secret deal with the west, etc. etc. While WWI Germany could be considered morally equivalent in that war to their enemies, the Nazis knew they were criminals and expected executions upon their capture. That's why they were so ready to let things go to the very end.

They were not only horrific to their enemies, but to their own people as well.