The market image is dark and gloomy, but that is to its benefit. The sun in the background evokes sunrise or sunset. Beginnings, endings, and transitions are hardwired emotional triggers in humans. We also respond to the sun. It's hard to tell which end of the day it's on, but both have significant meaning.
The size of the windows in the cupolas provide a sense of grand scale (even if they are only small decorative openings), so we see this as just a slice of a much larger city, and connect with that scale in our minds.
We couple the image with our experience. If we interpret it as sunrise, we relate to and actually experience that quiet, sleepy anticipation of a city before the hustle and bustle of the day gets underway. If we interpret it as sunset we see the opposite, and we feel the relief of a day winding down and revel in the peace of it.
Something about the horse image strikes me as unnatural, like you cut out the horse and blur the background behind it?
It also lacks a clear sense of scale and there is little to connect our experience with. It could easily be a bookend that you plopped down on a shop's counter and snapped a picture of to send a friend to ask if they thought it would look good on your bookshelf.
All that being said, I suspect that there are two disconnects in how you see the images and how others perceive them.
First is an awareness of your own effort. The market image probably came out of the camera almost exactly as it is. The way you talk about the image, you seem not to have planned the sun being directly behind that cupola. I expect you shot this image almost on pure instinct, so the perceived effort expended on this image is minimal in your mind. The horse on the other hand appears to have required a lot of work. Getting to a vantage point to shoot the image, contemplating how to frame it, probably spending a fair bit of time in post-processing trying to perfect it. Your perceived effort for this image is high.
Second is context. You clearly have a special connection with this horse, even if only a brief one. You find it beautiful and meaningful. You have little or no connection with the roofs above the market. It is far to easy to appreciate the subject of an image more than the image itself. This is why everyone thinks all photographs of their children are beautiful. No matter how bad the actual photograph is, they love that they can hold a moment of their child's life, so they love the photograph.
As artists we must frequently divorce ourselves from the process and our own context and learn to see the images with fresh eyes. This is an immensely hard skill to learn.
The most practical advice I can give on this matter is to put time between when you shoot and when you review the image. This was easier back in the days when we had to process the film than in our modern instant-feedback loop.
- Turn off instant preview and resist the temptation to look at each image as you shoot. I generally peak at the first image to ensure everything is set up correctly, then shoot the rest without so much as a glance.
- Sleep before you review them. This gives you a chance to clear the expectations from your mind and lets you look at the work with more objectivity. Wait a day at least, or a week or two if you can.
- Imagine you are evaluating another photographer's work when you do review them. I find it useful to speak aloud as though I am discussing the image in an art gallery showing, and I don't allow myself to say anything that I can't directly see in the image itself. (Your talk of 20 foot tall pillars would be right out.)
- Use post processing time as a guide. It is frequently—though not always—an excellent way to tell if am image is tremendously flawed. My rule is that if I wouldn't sell the image to the client as shot, there is probably no reason to spend time editing it. I wish I could remember who originally said it, but "Photoshop is not there to make bad photos good, but to make good photos great."