Art is often seen as a reflection of life, for good or for bad. Thus, it is not surprising that much of art contains elements that may be considered, well, downright depressing and full of conflict that grabs the attention. Everyone agrees that artistic creators should have access to the wide range of human emotions in the characters and events depicted within their works, and no one blames them for attempting to inspire the same range within their audience. Indeed, one of the things most disquieting about Plato's The Republic is how Socrates bans any emotional art from his supposed utopia. Without the influence of the tragic, we would not be able to experience some of the most critically-lauded works that have ever been created.
However, art is also seen as Escapism, a way to relax from your everyday stress, and rightfully so. Lots of blockbusters are wall-to-wall Scenery Porn that shows beautiful people in lovely locales. Everyone agrees that people have their own tastes in the media they consume, and have their own particular limits in how much fictional sadness and "interesting" conflict they are willing to put up with, simply because everyone has their own real life problems to consider; some more than others. It is not too much to ask for entertainment to be, you know, entertaining. If someone prefers a Warm and Fuzzy Feeling over how the realism and catharsis of a tragedy or Current Events-based dystopian literature that allegedly gets you to "think" (or enjoys either one depending on their mood), then more power to them. Only a very few want a world where everyone believes that "Silly Rabbit, Idealism Is for Kids!" and that the only True Art Is Angsty, and anyone who thinks happy art cannot be good art is certainly not understanding enough.
Angst aversion, then, is the situation occurring when these two conceptions collide: the personal tendency for a person to avoid a work once they hear it has a Downer Ending, and/or is generally filled with sympathetic characters that will be put through hell for nothing. This can easily happen when a normally lighthearted work is made Darker and Edgier, or undergoes Cerebus Syndrome — even people who normally have nothing against not-so-lighthearted fiction may lose interest if they had previously enjoyed the work for its lighter tone.
Angst aversion is likely the number two reason for Executive Meddling, behind the belief that Viewers Are Morons. People like (and will pay for) happy endings, and executives know it- though, granted, sometimes authors do need to be held back by calmer minds from throwing in a Cruel Twist Ending that will alienate their audience or suddenly making their work far darker just because they were going through a rough patch lately.
Compare and contrast Sweetness Aversion, in which audiences are repelled by the extreme cuteness of a work. This should not be confused with Too Bleak, Stopped Caring, where a dark work's stakes are insufficient for audiences who willingly gave it a chance to keep caring about how it would end, though both can be caused by the same general issue. Also not to be confused with Angst? What Angst? which is when a character exhibits no or little angst in response to what should be a traumatic event.