Everton completed the club-record signing of the Belgian on Wednesday night, paying a fee of £28million for the 21-year-old who scored 16 goals in 33 appearances on loan at Goodison Park last season.
Lukaku, who has signed a five-year deal, played just 15 times in three years at Stamford Bridge and had not enjoyed the best of relationships with Mourinho after disagreements about his loan spell last summer and then where his future lay in the long term.
But while Lukaku insisted he had no hard feelings towards his former club - despite not speaking to Mourinho prior to his exit - the Chelsea boss made plain why he had seen fit to sell a striker who had once been seen as the heir apparent to Didier Drogba when he signed from Anderlecht in 2011.
"Romelu was always clear that in his mentality and his approach he was not highly motivated to come to a competitive situation at Chelsea," Mourinho said.
"He wanted to play for Chelsea, but clearly only as first-choice striker - and at a club of our dimension it's very difficult to promise a player that status.
"That reduced immediately his desire to come to us. After that, Everton came with an important offer and because we want to be inside Financial Fair Play rules, you have to analyse these situations."
Lukaku said he had left Chelsea without speaking to Mourinho.
"I spoke with my agent and then from that moment on I left it up to him as I was on holiday," he said about the move to Everton.
"I made my decision and then didn't speak too much about it."
Asked whether he had spoken to Mourinho at all immediately before his departure he added: "No - I think he was busy with the game (against Vitesse Arnhem on Wednesday night).
"Chelsea is a big club and sometimes it is difficult for young players to come through. I want to have a good career and I didn't want to spend 10 years on the bench, I wanted to make the best out of every year.
"This is a club with a lot of ambition and it's the right place to be. You don't have to have regrets in football. Choices have to be made.
"Roberto Martinez will be one of the top managers in the world and for me to work under him now is the best thing that could happen for me."
Martinez was a key figure in persuading Lukaku to return and the board to break the bank to secure him and Lukaku admits things were totally different during his loan spell compared to his experience at Stamford Bridge.
"I don't have hard feelings against Chelsea. They gave me the opportunity to come to England," he said.
"But last year was a very good year and after a short period of time I had the feeling it was the best place to grow as a young footballer.
"The manager has been very influential. He is someone who knows football, someone who I can develop under as a player and make us better as a team - and that is something I need as a young footballer.
"I needed to be in the place where it felt right and here I have the confidence of all the technical staff, the trust of the supporters as well and I can grow and develop.
"I have a very good relationship with the players and staff so for me this is the place where I belong."
Martinez said they had worked long and hard to get the deal together but it was more than worth it.
"It is not just one of those important days for the season but it is a very significant day in the history of our football club," he said.
"It is fair to say Romelu was our number one target to bring in and the pursuit was relentless but it had to be a record transfer and it is worth every single penny and effort we put behind it."