The reason why is there is a plus sign in the middle of the negatives which results them being the parenthesis. -15 + -46 + -29 will always be adding the negatives. If it was two negative signs, -15 - -46 - -49, then you make the -46 and the -29 positive because a negative times a negative is always positive.
(-number) + (-number) = a higher negative number
-number + a positive number = the negative number subtracting from the positive number
