My daughter asked this question. I answered "because we like cuddling together because we are married." I went on and did say that we love each other and said that it is very important for your M. and daddy to spend a lot of time together to help nurture our relationship. I mentioned how we choose to spend our lives together and that sleeping together in the same bed is one of the great things about being married...
Love is NOT a bad thing worth mentioning!! Children should KNOW that M. and daddy love each other. It has NOTHING to do with lack of love with your daughter. J. mention the different ways you show you love her, if she shows a sign of that reason bothering her.
Not one of my four daughters, have had a problem with not sleeping with us. We had them right next to us when they were breastfeeding infants, but we never had a problem with them trying to come in our room to sleep with us- ever.
My daughter asked this question 2 years ago. Her sisters heard the answer as well. My daughters are 3, 4, 7 and 8.
But also, you could go for the short answer of Rachel's below, but it sounds like she's the type of kid that would like to hear more about WHY...