Sometimes the best thing to do is let someone down. We are all finite and while disappointing people is painful lying to someone by setting the wrong expectations is worse.
We set other people's expectations about us through our words and actions. All of these expectations can drastically change at any time, but engaging with a world where nothing can be taken for granted is overwhelming and impractical Doing things we do not want to and do not think we will ever want sets the wrong expectations about who we are and what can be expected from us.
The longer we do and say things we don't want to the more we perpetuate and solidify these expectations magnifying the disappointment and loss when we are forced to break the illusion. We can save ourselves and those around us from pain by simply rejecting some of our social nature and accepting we will disappoint people.
While belonging to communities and societies has enabled us to change human lives as much as we have, our social nature developed when communities were much smaller and experience was much narrower. Under those conditions it was more important and possible not to disappoint everyone around you. Now, though, we can be better to ourselves and the people around us by sometimes being disappointing.