In the first definition, retrospect is used as a reference to something similar in the past. This could be an idea, an event, an object, a person; it could be most anything. In this sense, most everything is in some way or another retrospective. This connection to the past is the key. Because we (as people) are inefficient long-term planners, we rely on this connection to help predict future outcomes. In doing so, we look back on the past, and make connections in retrospect. We consider this a wise thing to do, as it is really the only basis we have to make decisions on.
The second definition is not far from the first, being the act of looking back upon the past. This idea of retrospect is more contemplative than the first. It takes those connections established by the first to the past, and analyzes them. When one looks back in retrospect on their actions, they consider the outcomes of their actions, the lessons, the ramifications, their potential meaning and value. In this action, there is the idea of finding the relevance of the past in the present, and what very well may be the near or distant future. This sense of retrospect draws parallels with reflect and the act of reflecting. The difference between the two usually depends on the scale of the length of time being contemplated. Usually, retrospect is used to look back upon singular or similar occurrences, such as the late-night snack you just had, while reflection is usually used to look at an idea, fact, or timeline as a whole in much deeper thought, such as that whole winter you couldn’t stop snacking and why. However, the line is not always drawn here, and the two terms are often used interchangeably.
Not only is the retrospect the looking back — the reflecting on the past — it is the consideration of the past in lieu of the present. There will always be elements, roots, hints of the past in the the present, and retrospect attempts to assess the relevance of previous facts and opinions. The notion of looking back on something in retrospect usually carries connotations of a positive or negative experience, and the construction of opinions on the matter.
“reflect, n. : Ooford English Dictionary,” last updated September 2009, http://www.oed.com/view/Entry/160910?rskey=UK2ysX&result=1&isAdvanced=false#