Why did I just randomly start crying?

Nothing is wrong. I went over to my dad casually and then we looked at each other, i started tearing up and he did too and there were no words. I am like friends with my parents, we even talk to each other like we are pets! It’s very cute and intimate. This has never happened to me before. What was this all about? And even if I cried, why did he also cried a second after? I doubt he even noticed me crying.