Alonso exited Anfield last summer and joined Real Madrid in a deal worth around £30million.
The Span international admits his mind was made up 12 months earlier when The Barry deal failed to materialise, as Alonso felt he was not wanted by Benitez from that point.
"The pursuit of Gareth Barry took some time for me to accept and, in all honesty, it changed the course of my future," Alonso told The People.
"I did get over it but it did take time for me to accept what had happened to me.
"Leaving Liverpool was one of the hardest decisions of my life, but after what happened with Barry I knew in my heart I had to tell Rafa that I wanted to go to Madrid when they came knocking at the door.
"I was still fairly young when I joined Liverpool and I had not only grown into the player I'd become there, but also the man I'd become."
Liverpool's form has dipped this season and many put this down to Alonso's departure - but he does not think that there is a major problem at Anfield.
"People talk about this Liverpool team but it is more or less the same one that was challenging for the title last season," he said.
"Liverpool have been very unfortunate with injuries this season, but they have started to put wins together again and I am sure they will secure a top-four place.
"When you have bad results it is not always easy to get back to winning ways, but they have managed to do that and now it is time for them to look upwards.
"They have a great squad and finishing in the top four is a must for them. I think it is something they will achieve.
"I would not offer advice to any of the players because they don't need me to tell them what to do, but I do offer them my friendship."
Alonso still has a special bond with Anfield and said: "I still see and speak with all the Spanish boys at Liverpool, but I also speak with others there. I am in regular contact with Stevie Gerrard and Jamie Carragher. I spent a lot of time at Liverpool and in those two I have made friends for life."
Alonso insists there is no ill-feeling between him and Benitez.
"There is no questioning that he is a good manager who can still bring success for Liverpool," he said.
"He doesn't operate the same way to a lot of other managers, but you can't deny that he is successful in what he does. He keeps his distance emotionally - you are the player and he is the manager and that's the way he likes it. You have to accept that.
"I will always respect Rafa for what he has done in football but, more importantly, I will always be massively grateful to him for the chance he gave me to join Liverpool.
"The club gave me so much and I made some great friends - and the Liverpool fans are beyond excellent.
"It was such a hard decision for me but I knew in my heart it was the right time to leave.
"It will always be a special place for me, and when I can I will always go back and watch the team and see my old friends."