Wan-Bissaka spent five years at United after being sold by Crystal Palace, before returning to London with the Hammers.
He told the London Evening Standard: "I came back as a man. More mature, more grounded, more focused.”
Asked if he left Palace too soon, Wan-Bissaka also remarked: “I’ve thought about that.
“It could have gone any way. I could have told myself it was too early but the other side of me was asking, ‘Is this opportunity going to come again?’
“So, I thought, let me just take it. If it doesn’t work out, it doesn’t work out. At least I’d have tried and given it my all. That was my mentality.”
On West Ham, he added: “I spoke to the club and they thought this opportunity was best for me.
“They gave me some time to think about it. In the moment, I didn’t, but I spoke to family and friends about it and we all decided it was the right time.”