The reason people lose interest in photography is usually related to the type of images that they are ingesting. There is something that I crudely call "the psychology of the shot." This is what I use to describe the elements within a photograph that are appealing or unappealing to the viewer. It's related to aesthetics but goes farther than that because the types of images that a person finds pleasing or displeasing often has as much to do with the personality and intellect of the viewer as the image itself.
The physical subject matter contained within an image can provoke a response from the viewer both in terms of the senses and the intellect. Images that rely solely on sensory elements to generate a response can become boring very quickly, whereas the intellectual elements of an image will generally remain timeless.
An example of a sensory element within an image could be the superficial appearance of the actual physical subject. For example, some people like cars and enjoy looking at pictures of cars. These people enjoy pictures that contain cars that are pleasing to their eye and reject pictures that contain cars that they consider ugly. This is a strictly sensory response. Some people like pretty girls and enjoy glamour pictures. Exposure, sharpness, color balance, lighting, bokeh etc when viewed solely for their superficial qualities and devoid of any symbolic meaning behind their use become sensory elements. Almost EVERY critique and photo sharing section on the web relies on sensory elements in order to succeed or fail. Genres of photography are actually defined by shallow superficial qualities like people, landscapes, and wildlife. When a photograph relies on the senses alone to be successful then it's lifespan is usually short lived because the viewer that recieves sensory pleasure from viewing those images needs a constant stream of new stimulation to prevent from becoming bored.
Intellectual elements contained within an image can provoke a more reasoned response from the viewer and can have the effect of remaining enjoyable for greater periods of time, possibly even an entire lifetime. People that are interested in distinguishing these elements might want to start reading up on Aristotle and Hegel. Some of these elements can include symbolism, lines, form, repetition, duality etc. These elements can be more difficult to recognize and not every viewer will notice them. In fact, most viewers will not be aware that they are even there, so these elements can often only appeal to the trained observer. Bresson could be an example of a photographer that created images relying on intellectual elements, like strong lines, and that's one of the reasons that he is still popular today.
A good photographer should know how to marry both sensory and intellectual elements together. For example, he may choose a particular color scheme not just because of it's sensory appeal but also for symbolic meaning. Or he may choose a lighting scheme both for it's beauty but also it's ability to accentuate strong lines.
I also believe that there is a message contained within an image that can provoke a spiritual response on behalf of the viewer. An image can provoke lust, envy or a carry a mocking tone towards the subject matter. These types of messages will appeal to the darker spirits. An image can also emphasize noble things like love, dedication or promote the family. Images that contain dark messages will provide a momentary satisfaction to the viewer that shares the same dark spirit, but they cause depression in people that do not share the same dark spirit.
In order for an image to be considered great art, then it really has to contain sensory and intellectual elements while also appealing to the noble spirit. Images that rely only on sensory elements are shallow. Images that provoke dark spiritual responses are degenerate.
I'll be the first to say that it's a wasteland out there. There's a ton of photography to look at but almost all of it is either shallow or degenerate (or both.) I think it's a reflection of our culture in general and won't change until the culture changes too.
