You have probably seen things like this piece, from Jonathan Freedland. He starts by asking why it was that the Brexit referendum result did not result in some sort of euro-fudge rather than a real Brexit. Then he asks, "How did May, who campaigned, albeit in lukewarm fashion, for the Remain side in last summer’s referendum, end up pushing for such a hard-core version of Leave?". Having asked those questions, he continues, "Any explanation has to begin with the parlous state of the official opposition to the Conservatives now in power."
That is British self-regard at its highest. Let's forget about Jeremy Corbyn for once. This sequence of articles from the FT makes it clear that it was the EU that decided on hard Brexit even before the Conservative Party had even decided who was to replace David Cameron:
"... it did not matter who won that leadership election, and what they thought Brexit meant. Senior figures in the EU had quickly adopted a position that they have stuck to since. Brexit was final, there would be no renegotiation, there would be no negotiation without notification, the exit would be “orderly”, the prescribed process was to be followed tightly, full access to the single market required acceptance of the four freedoms, and the EU27 would act in unison. And all this had been stated precisely and openly.
[...] The EU had already formed a view on what Brexit was going to be like for the UK, regardless of what any British government would want. In essence, if not in detail, the basis of Brexit had been set, and Theresa May was not yet even prime minister."
The EU (atypically quickly) decided that the deal on offer was all-or-nothing, i.e. 'hard' Brexit or no Brexit. That was the choice facing May. Even if she had wanted the softest or all possible Brexits, it was not on the table. The referendum result said Brexit; the EU said that meant hard Brexit; and the only question for May was whether to accept the democratically expressed will of the British people or not. She did not choose hard Brexit - it chose her.
Let me put it another way. If you are or were Remainer you are probably sympathetic to the argument that in any negotiation between the UK and the EU it is the EU, as the bigger and more powerful party, that holds the trump cards, and that the UK will just have to take whatever it is that the EU has to offer. Well, take that as an indication that the EU is only offering a hard Brexit.
An interesting question is how and why the EU has been able to - and has decided to - act so quickly and firmly in this matter, when it typically acts more slowly and with much greater room for compromise. Views differ. I may offer mine on another occasion.