Happens all the time. I think it's a combination of long distances, atmospheric conditions (dirt particles, moisture, light scattering) and limitations in dynamic range of digital cameras.

Your shooting conditions seemed to have been quite difficult with big difference in brightness between the upper and lower parts of the image. The image is also a bit underexposed, making it look flat and dark. Also quite typical for this kind of shot.

Use the Levels tool to correct black point and white point for more contrast and brightness. Just watch the cloud details, so that you don't overdo it.

You can also add more contrast with the unsharp mask tool. If you're using Photoshop, try these settings:

Amount: 10-30%
Radius: 50 pixels
Threshold: 0

This should give the image a nice contrast boost. You could also try using shadow recovery tool to brighten the forest area in the foreground.