He told BBC Sport: "It is a boyhood dream, but they never come true normally. This is the exception.
"It's ruled by the heart. This is not a financial investment for me. If I wanted to make a financial investment, I would buy another chemical company.
"It's very much taking on one of the great challenges in the world. It is a very worthy challenge and very exciting.
"The only reason I got involved is because I want to see Manchester United restored to where it should be. I might be biased but I believe it is the biggest and most well-known club in the world. It should be playing the greatest football in the world.
"It has clearly been a difficult 11 years since Sir Alex Ferguson and David Gill retired. It's not switching a light switch. It's not just a simple short-term fix. We have to walk to the right solution, not run to the wrong one.
"The short-term issue is we want to get into the Champions League. It's a two-to-three-season challenge to get the organisation and environment right, to get the performances on the field right and win football matches. That involves the design and structure of the organisation being correct.
"In the last 11 years, Manchester United have had a lot of coaches and nobody has been very successful in that environment. That says to me there is something wrong with the environment. It is not constructive for me to blame anyone, it's just a fact. My focus is on how I change that environment to get the best out of the coach and squad."
On working with co-owners, the Glazer family, Ratciffe was upbeat.
"I only know Joel and Avram and they are, despite what you might read in the press, very nice people, very courteous and they are avid supporters of Manchester United. I understand the frustrations and the anger (of supporters) but I am looking forward not backward.
"Try to be patient and we'll try to build Manchester United back to where it should be, which is as one of the very elite clubs in the world. The key to it working is the relationship we will have with Joel and Avram, which in my view is a very good and trusting relationship.
"I think in a way it might have been the easier solution for the Glazer family to sell outright to the Qataris. To be fair to the Glazers, they thought the best solution was to sell to me because they thought United would benefit more from that."