Welcome to the wonderful world of printing @hampusk
Because I did some prints recently and they were a bit warmer and darker than I was hoping to.
Screens are shipped with a white point near the D65 standard, without going into the whole history of white balance, this point was set for television and all that, i.e. screen applications.
Pre-press people tend to use a warmer white point (D50) because paper is not as cold/blue as a screen is.
You have two solutions here, either calibrate your screen to D50 and hope that your paper is going to be close enough to that white balance or get an ICC profile for the paper that you will be using and apply it as soft proofing in Lr or Photoshop.
Since you also use your computer for other things than printing you should stick to D65 but make use of the soft profiling options in Lightroom.
NB. I am oversimplifying this to the absolute minimum to get you started. When I get ready to print an image I calibrate my screen to D50, softproof the image to a known ICC profile for the printer AND paper, print a test chart and then scan it with a colorimeter on an appropriately illuminated and controlled viewing surface…
This video gives you a good primer on why your images appear too dark: why are my prints too dark?
http://digitaldog.net/ is a very good ressource in setting yourself up for success when printing images.
But now it just feels kind of weird. For example, when I do auto exposure in Lightroom it still feels waaay too dark.
This might be because your images are really under-exposed, that your eyes are still getting used to your new screen brightness, or that your ambient light level are quite high. The latter is other the reason why other people screen brightness settings are meaningless to you because their ambient light level is most probably different than yours.
A good tool is looking at your histogram in Lightroom, it will be your best indicator as to whether a picture is properly exposed until you can just judge by eye and calibrate your screen.
A word of advice, Auto adjustments in Lr are just absolute rubbish. Don’t use them.
EDIT I just want to add that US$155.- for something like a colormunki display that will tremendously help you when bringing images to print is a small price to pay when you consider how quickly the price of prints rises.