Having gotten to some beautiful location, do you also sometimes wish that — especially when travelling — you were the only person there? How many times do we try to take photos with as little other people on it as possible? Maybe what is happening is that (subconsciously) we believe that the picture will be more special without other tourists in it, as in we were the only person capturing this wonderful moment / place.
While of course there are many situations where it is much more aesthetic to have no persons in the picture (also, from a compositional point of view, people usually draw a lot of attention to them, away from other potential subjects in the picture), fellow humans can also give a bit of life and fun to a picture, or a situation.
Take the picture above. I had just set up to take the shot of the Cathedral of Brasilia when a horde of tourist poured out of a bus and jumped into the picture to take their group photo. I almost got annoyed by what I felt to be a rather rude thing to do, but then I saw that those crazy young people actually give so much more liveliness to the shot (compare it to this shot someone else posted on flickr). In a moment of adventure, I asked if they all could cheer and wave and scream — and spontaneously they did!
So here’s something to think about: Tourists may also be your friend in pictures! Also, intentionally including people helps to give scale and perspective to some shots. Here are some more examples (1, 2, 3).