Sorry because I let you stepped on my foot

I am going down the stairs, and Alex is going upstairs. He accidentally stepped on my right foot.
"Oops! Sorry." I said.
I walked a few more steps, turned around and asked him:"You stepped on my foot. Why should I say sorry?"
...
"Oh, because it was my fault to walk my foot under yours. You never meant to step on my foot."
"Yes!" Alex answered with a big grin on his face.