Why Should the Sum of Masks / Kernel / Filter in Image Processing be One?
When you're trying to create a blurring kernel (namely a weighted mean of the data) you keep the mask sum as one.
This preserves the mean of the filtered image (think about the DFT, if the sum mask is 1 its DC gain is 1 which means it keeps the sum of the image which means it keeps the mean value of the image.)

When you're trying to apply a high pass filter you want to have gain of zero at the DC coefficient.
This is done by having the sum of the mask as zero (same reason as above.)