Historically, the relationship between Debian and Ubuntu has been strained at times. There are various technical and social reasons behind this discomfort in our relationship, and while there is still work to be done to ensure we are working effectively together, the relationship has most certainly improved in recent years. I think there are many reasons for this, again technical and social, but I think you can boil it down to a critical evolution in our relationship - we have learned more about how a large derivative (such as Ubuntu) and Debian insect, mirror, and vary in different ways, and this takes time.