When making decisions, the “pros” are much easier to deal with than the “cons.”  Your boss wants to see both.

This is another aspect of making sure that you see what’s missing.  

There is nothing more valuable than a review process that surfaces all the potential consequences of an action rather than just the best-case scenario.

There is no way to effectively wight the risks and benefits of an action if one is ignorant of the risk.

By presenting both the pros and cons, you show others that you have thought the problem through.  You will have more credibility.