Why might some presidents have used the veto power frequently, while others have not used it very often?

The veto power of a president is used when Congress passes a bill that the President believe shouldn't be passed. This usually happens most often when the Congress's majority party is different than the President's party. For example, if there is a Democratic President and a Republican Congress, the veto power is generally used more often. If there is a Republican President and a Republican Congress (or a Democratic President with a Democratic Congress), there is usually a smaller number of presidential vetoes because the President is thinking on the same line as the Congress (usually).