The 41-year-old Spaniard kicked the silly season into high gear after last month’s Hungarian Grand Prix when Aston Martin confirmed him as replacement for retiring four times world champion Sebastian Vettel.
Alonso had been expected to stay at Renault-owned Alpine, the team with whom he won his two titles in 2005 and 2006 and made his Formula One comeback last year.
"We were moving around in different things and we were not maybe agreeing on the principles," he told reporters at the Belgian Grand Prix.
"It’s not only what you agree in terms of duration of the contract, it’s just also the trust that you feel and how you feel wanted in a place.
"I felt that it was the right decision to move to Aston because they seemed to really want me and appreciated every performance I was putting in the last two years."
The oldest driver on the grid is 10th in the standings having scored nine times in 13 races. He swaps a team that is fourth in the championship for one that is second-bottom.
Aston Martin, owned by Canadian billionaire Lawrence Stroll, have big ambitions and are investing heavily in infrastructure, facilities and people.
Alonso, who has agreed a multi-year deal, was understood to be unhappy with the length of the extension offered by Alpine.
The French manufacturer had planned to loan Australian reserve and Formula Two champion Oscar Piastri to Williams but that all changed when Alonso announced his move.
The 21-year-old was immediately announced as the Spaniard's replacement but then said he would not drive for Alpine next season and is now expected to join McLaren.
Alonso, who is friends with Piastri’s manager and former F1 racer Mark Webber, said he was as surprised as anybody by the Australian’s snub of Alpine.
The Spaniard also said he informed senior figures in the team of his departure before it was announced, suggesting principal Otmar Szafnauer, who said he found out about the move from Aston Martin’s press release, was kept out of the loop.
