The total probability of a set of related events sums up to 1 because the probability of each individual event in the set is a fraction of the whole. For example, suppose you flip a fair coin a large number of times.
If u add probabilities of all possible outcomes that should be one, because classical definition of probability is number of possible out comes divided by total number of outcomes. When you add all probabilities numerator and denominator are equal so answer is one.
