Here’s a good question…
“When I first load my pictures into Lightroom, they look great! But then the preview changes to something not so nice that needs to be tweaked. Why is Lightroom changing my files? I want to keep the original settings!”
The first image that you see is the preview jpg which the camera created. If you’d shot jpg in camera, that’s how your picture would have looked. Then Lightroom finishes reading the raw file, and applies it’s default settings. A raw file is a linear file, not an image, so settings must be applied to turn it into an image. The camera manufacturers don’t share their information, so Lightroom applies its own defaults.
If you want the raw files to look like the preview jpeg, you have to play with the settings until you find a close match, and then save them as a preset to reuse on future occasions. The easiest way to do this is to shoot a series of different situations in Raw+Jpeg and try to match the raw file to the jpeg. Once you’ve come up with settings you’re happy with, you can either save these as a Preset and apply them on import, or you can change the default settings for that camera. More on that another day!