Could it be that we become so entrenched in our approach to photography that we forget how to have fun with it? Like a pilot preparing for take-off, every shot includes a long checklist of technical and aesthetic minutia that has to be considered before the shutter button can be pressed exposure, background, direction of light, shadows, depth of field, shutter speed, fill flash, diffusers and/or reflectors, filters, subject parallel to film, and forget about trying to capture a great image in harsh light.
There in, I believe, lies the part of the problem. The pursuit of the same type of subject, using the same repetitive measure of success, can ultimately lead to the stagnation of artistic intuition and diminish enjoyment of the activity. Before we know it, every shot we take becomes nothing more than a regimented process governed by a mental list of dos and donts. While mastery of these techniques and procedures ultimately makes us better photographers, the absence of the "fun factor" will most certainly be evident in our results.
I, for one, am getting bored with architectual themes and plan on having some fun one evening and hit the subway. My equipment will be simple; digital cam, 28-70mm lens and a monopod. It's something different for me and I'm taking a fun, lackadaisical approach to this. Like you, I need a change.