So that (almost) everyone has an opinion and so one branch of the government doesn't get too overpowered.
It is something that causes someone to get sick
The power of the President to refuse to approve a bill or joint resolution and thus prevent its enactment into law is veto. This veto can be overridden only by a two-thirds vote in both the Senate and the House. ... If this occurs, the bill becomes law over the President's objections.