Setting aside the national interest and trying to think like a politician...
Brexit is a massive blame bomb and nobody wants it to go off in their hands. You’re either failing to honour democracy by preventing it or responsible for the consequences of delivering it. So it is key to shape perceptions so that it seems like somebody else is making the decision.
Boris Johnson’s plan seems to be:
1. Make the EU an offer they can’t accept
2. Hope parliament (mostly Labour) block no deal (or, less likely, take a share of the blame for letting it proceed)
3. Go for a general election saying he did everything he could but that he was stymied by the perfidious EU and treacherous socialists, winning over Brexit party voters, while the remain vote is split
4. Big majority, 5 more years and room to manoeuvre - he won’t have thought any further than that
Labour, seeing this coming, won’t want to be stuck on point 2 - either voting down no deal and denying Brexit voters their dream, or allowing it and being consigned to electoral irrelevance as many of their voters switch to the Lib Dems. They’ve been banging on about a GE for what seems like forever, but a no deal versus remain referendum first would now be more attractive for them imo. No deal is sufficiently different from what was pitched in 2016 to justify another vote, again imo. And either Boris/no deal loses or Labour dodge any share of the blame if the people decide to take the no deal plunge, and they can push for a GE again if/when it all goes a bit shizzle.