The idea is that, even though the bond itself (double or triple) is stronger than a single bond as a whole, we must consider the components of a double or triple bond. A double bond consists of one sigma and one pi bond and a triple bond consists of one sigma and two pi bonds.(You will study more about them in higher classes). The pi bonds are weaker than sigma bonds and easily breaks. So, double and triple bonds can easily break their pi bonds to become a single bond.