When M was 3 she got in a groove of asking periodically for me to tell her some of the ways that I know I love her. Sometimes this was a spring board for moralizing. "Some of the ways I know I love you is that I make sure you are buckled up in the car."
Mostly it was an incredible opportunity to explore what it means to love people, and remind her that she is loved.
We would also discuss the ways we knew she loved me, how dad loved her, how dad loves me etc...
This was the beginning of our serious conversations. Still enjoying this little girl with all her questions and wonderful ideas.