Answer :
There were always two consuls in power at a given point of time. The right to veto the other's decision was seen as a measure to prevent the abuse of power by the other consul.
The reason why consuls were awarded the power of veto was to avoid the abuse of power. This is because within the Roman Republic consuls were granted both executive and judicial powers, as well as being the supreme commander of the armed forces in times of war. To prevent one consul from acting against the will of his peers, or abusing his power, the second consul was granted the right to veto the decision of the other, and bring them to a review by the senate.
