Why is it so hard to passively stalk my friends' locations?

I feel terribly guilty when I visit a new city, post photos of my travels, only to have a friend say "Hey! Why didn't you let me know you were in my neck of the woods?" Similarly, if I bump into an old acquaintance at a conference, we both tend to say "If only I'd known you were here, we could have had dinner together last night!" I do enjoy the serendipity of events like FOSDEM - randomly…