Yeah the simple answer is just that the digital scale has a maximum value of 0dBFS, so everything you do will have a negative value.

Has nothing to do with mixing in particular, but to do with the format (and so which dB scale is being referenced, because there are a number of them for different situations). If you were mixing in analogue (i.e. dBu, dBm, or dBv scales) you would still see negative values but you would also have positive values too.