In 15:18 it says that when you release the servant, it should not be difficult for you... and Hashem will bless all that you do".
Why? Why should you be blessed for letting him go? You bought him for 6 years and the 6 years is up. He is a free man. Why do you deserve a blessing for this?
Some meforshim say, including Rav Hirsch, that this blessing is not referring to this passuk, rather to the previous passuk of drilling his ear, that you will get the blessing...
Not to argue with the meforshim, but to take it more along the actual pshat - the passuk says the blessing on the verse of not feeling bad about letting him go. I think the Torah is telling us an important lesson - that even though it is not up to us, even though it is something we have to do, we still might not want to do it.
It is very natural that the owner will feel bad - he has gotten close to the servan perhaps, he has gotten used to having a sevrant take care of his needs, etc. Even though he has to let him go, he might feel bad about it. He might even try to prevent it, or convince the servant to sign on for more time, or maybe not at all but still feel bad abou tit.
The Torah is giving the blessing to the owner for, despite the natural feelings, not doing anything to prevent the slave from leaving.