Missing out is not necessarily meaningful. FOMO is the anxiety caused by missing out. which may cause you more pain than actually missing out on that thing.

Ah this is a great point - worrying about something that could happen as opposed to worrying about something that did happen, but also leads me to the conclusion that the very idea of “missing out” is pure fear, a narrative about what other people are doing and the self-imposed expectation that you should be doing the same thing, right? Maybe there is no MO without FO?

If you choose to miss out then watching other people having fun on snapchat is no problem. The anxiety and/or depression kicks in when you see your friends or social group having fun together without you when you never were invited in the first place.

Emotions just are. Thoughts come and go, but we may or may not pick the ones on which we loiter. If undesirable emotions ebe when we loiter on certain thoughts, repeated behaviors may emerge. Let's do this to get rid of that emotion, your brain might say to itself.

Missing something, like your sunglasses, makes you wonder where they are, look for them, call people asking if you left them over, etc. This is worry, which is a light version of fear.

"Missing out" isn't like looking your sunglasses but still makes you wonder if where you are isn't where you should be, which is still a type of anxious behavior which will lead to repetition if the emotional response is allowed to go on unchecked.

Check out "Plutchik's wheel of emotions" for emotion groupings.

Perhaps "feeling of missing out" could work as a good description? If more reminds one of things that one is "missing out" on, one probably feels more that one is "missing out" more than otherwise would, given equal amounts of things that others are doing that one is not doing?

colloquially this is how the acronym is used though (doesn't have to make sense)