I asked a woman I was getting serious with a very similar question.
I asked her to give me three (3) reasons why she liked me.
She couldn't.

I took that as a very bad sign for the future of our relationship. If you can't answer a simple question like that, or you have to reach too far for the answers, then why are you even with the person? It's not a huge intellectual exercise where you have to sort through a bunch of psychological or philosophical reasons or support. Just give a quick, honest answer of the first few things that pop into your head.
"I like your smile - it makes me want to smile."
"I like how you treat your dog/cat/pet."
"I like that we can talk about the same TV shows."

It doesn't have to be,
"I like your stance on the neo-economic principles Johnson proposed in his latest study."
or
"I like you because we connect on the deepest emotional experience I've ever felt in my life."

In other words, don't overthink a simple question. Dude could just be curious. It's never wrong to want solid reasons for something or someone you care about. Imagine if this was totally serious, like giving wedding vows. Imagine
"I, uh... want to marry you and love you and have lots of sex on our wedding night because... uh..."
Compared to
"I want to spend the rest of my life with you because I know that you are responsible, you'll always be there for me, and I will be there for you. I want to love you because I know you will make a great mother some day because of the way I've seen you treat your dog. I love the way you scrunch up your nose when you laugh, and I want to make you laugh, laugh lots..."

I'd put my money on the second marriage lasting a lot longer than the first. (At least to the end of the ceremony!)