Sometimes we may be left out of a situation, and sometimes we may choose to be left out. Other times it may be a mix of choice and circumstance.
Why would you choose to be left out? In the past, I've chosen to be left out when I'm unsure of what's to come and when I have little trust in the people or process. In my professional life, sometimes I would not get involved in a project or endeavor if I didn't understand, trust or believe in the project's aim or effort. The same has been true in my personal life. I may avoid an event or meeting if I'm not sure it is a positive endeavor or if it is an endeavor I am unprepared for.
Yet, when we choose to be left out, we miss out on the event first hand and are relegated to finding out about what happened second hand which is never as accurate or trustworthy as seeing or engaging in an event on your own. For this reason, we have to be careful about choosing to be left out.
At times, we are simply left out by others, not by our choosing. This happens to everyone for all kinds of reasons. If the event or effort means a lot to us, then we may want to inquire as to why we were left out to understand what's going on, but in general, being left out is not a big deal, but a matter of circumstance instead. For example, we may be left out of social events due to proximity, relationship, numbers or timing--the circumstances led to the fact that we were not invited, and little more. For this reason, it's important to not take being left out too personally.
When and why do we leave others out? In general, I never think about leaving people out as much as I think about who I can include. In general, I try to include most people when it comes to family and friend events, and the most common reason for leaving people out is numbers and the event type. For example, an intimate dinner party includes a few while a big social gathering includes many. If I'm planning an event with others, everyone gets to invite some people, but rarely can we all invite everyone we'd like to invite. And if it's a very personal matter, I'll typically keep the numbers small--no one wants to share their most intimate life experiences with a big crowd.
Left out is an experience we all face in life, and like all life experiences, there's no single reason or decision why this occurs.