Why is my pork roast always so dry?

I've given up on my slow cooker for everything but pulled pork which is covered in sauce  

All other large pieces of meat are roasted in the oven using a meat thermometer ... mine is digital and is the best $22 I've ever spent   

If you are using a pork loin roast, it does not have nearly enough fat to go through long cooking like that and be tender. 

A roast is much better in the oven (at 350), and pork only needs to be cooked to 145 (+ rest 3 minutes). Don't overcook it or it will be dry. 

The short answer to every "my meat is dry" question is: you over cooked it. Every time. No matter what. If it's dry it's not how you cooked it, it's what temp it got to. Over cooked meat is dry even if you boil it in water. Use a thermometer and only cook to the recommended temperature.

sometimes too much liquid can cause that-it's important to keep that lid on the pot and resist the urge to keep checking. I also find it helpful to sear the meat before putting in the crockpot and don't try to cook something that is too lean in it. you need some fat to keep it moist

I have pork tenderloin in the crock pot as we speak. Covered with Caesar dressing and on low for 4 hours. This is how I always do it and it turns out great.

You are not always boiling meat in a crockpot, no more than you boil meat in an oven when you put it in a cover dish.

I cut huge onions in half and set my roast on top of them. Plus whole carrots and celery. It helps keep the roast out of liquid until it browns a bit.

some cuts of meat are just dense and super lean. it could be submerged in liquid but when you cut into it, what little juices are inside will drip out and dry out the meat.

It doesn't matter what cooking method you use. It's about the temperature you STOP the cooking at. It took me years of watching various cooking shows and finally giving in to using a meat thermometer. Once I started cooking to the right temperature for the meat I"m cooking, I stopped ending up with sawdust for dinner. And actually taking it off/out of the heat about 5-8 degrees lower than the ultimate temp I'm after. If I want 145, I pull out to rest at 138-ish. It "finishes" up to 145 resting on a plate near or on the stove (sometimes covered with foil). Resting is another crucial step.

99% of the time, when using a slow cooker, this means FAR, FAR, FAR less time than most recipes recommend, even on "low". I'm talking that if the recommended time is 6 hours on low, I likely find it done right at 3 hours. Sometimes 2.5 hours!

I agree with you for lean and/or tender cuts of meat, but for tough cuts that rely on a long, low and slow method to break down the collagen into gelatin to achieve tenderness, just achieving a certain temperature is not sufficient.  You have to raise the temperature of the meat past 160 (180 is optimal) and hold it there for a long time - which is exactly what slow cooking does.  This will dry out the meat fibers, no two ways around it - the illusion of juiciness is maintained either through the rendered fat and collagen that clings to the meat, or the liquid that the meat is cooked in, or a combination of the two.  But failing to allow enough time for the collagen to break down means your meat will be tough (again, unless it was a tender cut to begin with).

Here's some information about the science behind this.

It took me ages to convince my husband that his brisket was not done just when it reached 180 - it needed to stay at that temperature, in a moist environment, for several hours, to be tender.  When he started listening to me, his brisket started being something we could serve to other people.