Help with the 'why'

My husband had an internet long distance affair many years ago. While I have tried to move past it, I just can't wrap my head around the why. He had always said I was the best person in his life, the only one who loved and supported him. But yet, when I was grieving the loss of my mother, he decided to start a relationship on line.

The hard part is that he told someone intimate words like I love you to someone other than me. I take my marriage vows seriously and this felt like total betrayal.

We are good now. My husband does acknowledge this is his biggest regret of his life, but I still don't understand why. Why did it happen in the first place?

Can anyone shed light on this? Any ideas, comments, or opinions?

AG