How can I understand why my partner cheated

Dear Lori and Jeff,

My husband and I have been married for over fifteen years and have three beautiful children. We’ve had our ups and downs over the years but I honestly thought things were going well between us. About three months ago, I found out he had an affair with an ex-girlfriend. He says it’s over now and was a huge mistake and that he still loves me and wants me to forgive him. I do still love him and want to make things work, but I still can’t wrap my head around why he did what he did. Can you help me understand?

Signed, Can’t Move Forward