Legislative-executive conflicts

In your textbook in Chapter Ten, Congress and the President, under the section on legislative-executive conflict, it includes the following statement:  

 “Legislative-executive conflicts were evident in 1789, they are present today, and they can be expected in the future for at least three reasons.” (p. 301-2.)

 What are the three reasons?

 Please list and elaborate on each.

Length: 75-250 (enough to succinctly answer the question)