Lets say that someone has made a short-film and they have a scheduled a screening at a local theater for their friends and family to see their latest masterpiece. The friends and family (unless they are immediate friends/family) are likely to perceive the whole affair as just going to see this "little" movie and have no concept of how many hours (likely several hundred hours or more for a short film) were put into the few minutes they are going to watch. Because of their lack of understanding of what goes into any movie, the significance of the screening for them is likely lost amongst their thoughts about what they're doing before or after they sit in their seat.
Too many times I've been a bystander viewing with horror a filmmaker who holds a public screening (sometimes where people actually have to [gasp] pay for their ticket) only to have a couple people show up to see their film. What went wrong?! Was the movie just not any good? That can't be the problem since no one has seen it yet, right? Most filmmakers spend so much of their time, energy, and thoughts on the actual making of the movie that they forget there's an even harder effort in front of them to market the movie.
The creme de la creme of festivals for most filmmakers in the US is Sundance. It opens next month in Park City, UT and I'm betting there's going to be more than one director or producer who's ready to cry when its over. Movies face overwhelming odds to even get into Sundance and there's always someone who assumes their entry guarantees them a capacity crowd to see their movie. Not only is this grossly naive, but the costs to attend festivals for the filmmakers is usually pretty high without them spending any money to market their movie at the festival. They usually don't get multiple shots at "getting it right" within their limited windows at each festival.
How or why would anyone want to attend a screening of their movie at a festival and be confronted with the horror of an empty theater? It takes significant effort to "get the word out" for any movie - whether at a festival, at a single engagement at an indie theater or a DVD party at some local venue. A very wise and experienced producer friend of mine once said to me, "no one is going to care about your movie more than you, and no one should be working harder to get people to see it than you."

In closing, the next time you're invited to attend a screening of someone's film, remember that it likely contains a piece of their heart and soul. Whether you decide to blow them off or make an appearance is obviously up to you. I would ask you though... for an event that might be as sentimentally important as a wedding, can you really afford to blow that off?!