Good question. Think of this as a general heuristic or a rule of thumb that works only when the first term usually x^2 is 1. I'll use some examples:

X^2+5x+6 both are positive so the answer will have two positive signs

X^2-5x+6 both are negative. 

X^2-5x-6 this with the term x^1 will have a positive and negative symbol. It is important to note the larger number's sign will dictate the sign of x^1

Remember that the reason it works this way is from the "FOIL" process