Tired from swimming

I remember stuff from when I was 4-5 years old. When I was 5 I remember my dad taking me swimming in the back yard, after swimming I was always so tired and sleepy. My dad and I would take our trunks off and wrap up with a towel and sit out on the porch in chairs and drip dry. I remember sitting in my dads lap with my back to him and I was layed back passing out. While I was falling asleep I kept feeling my dad move around but I didn't think anything of it, I thought he was kind of bouncing me but when I opened my eyes I realize now what I was seeing. My towel was on the floor and my dad's was open, he was reaching over my right hip and between my legs to stroke himself while we were both fully naked. I remember it didn't take long for him to cum, I watched it shoot up and over onto my genitals and stomach. When he was all done he wiped me off with his towel and wrapped us both up again. I remembered this finally when I was 11 , and other stuff. But for some strange reason I get so turned on when I think about this. Does anybody know why??