Donald Trump craves acceptance and adulation. Much of his 45 year history at the literal and figurative center of Manhattan has been driven by a profound drive to be accepted as a peer by the city's money elite and his general failure to achieve that. The drives the convoluted mix of neediness and populist, anti-elite grievance and grandstanding we associate with him. That's the most salient thing about Trump's candidacy. Even though Trump is a thoroughly New York creature, an elite of elites and a plutocrat, someone with virtually no connection to the people he energized to the polls, he had nevertheless an experience of anti-elite grievance that made the connection possible and galvanizing.
Mitt is the widely respected elite who looked at Trump, regarded him as trash and told him so. For all the difference, for all the non-New York-ness of Mitt, it's the kind of rejection and insult that we can see as formative and driving influence on Trump's life.
Mitt is the real thing. Many of us see him as a touch dorky or square. But Mitt is widely respected even among political opponents. He has political pedigree, great success in business. I think there's part of Trump - a big part - that, as much as he'd like to humiliate Romney, would really like him to join him, join his team, accept him.
Look at Trump's loyalists.
They are virtually all has-beens, unknowns, hotheads who the media and the political party elites see as embarrassments or jokes. Now, maybe Trump saw something in them these guys didn't. He won the election after all. So maybe so. But still, they're desperadoes and has-beens and unknowns. This applies to Manafort (cashed out long ago, damaged goods), Lewandowski (ne'er-do-well in the business), Conway (was a big deal in the 90s), and all the various press spokespeople and handlers. In a different way it applies to Rudy, Newt, Huckabee. To paraphrase Trump, when they were sending Trump loyalists and surrogates, they weren't sending their best.
The desire to humiliate is probably much the stronger with Trump. But I think Trump would like Mitt to validate him too.