Laziness at work

So recently I have been thinking about my job. I have worked here for over two years now, and have been awarded for my hard work a few times. But I was thinking today as I am trying to get someone to work for me, that I have taken classes for just about every person I have worked with when they have something come up. I understand that that happens, and I usually feel bad that no one else will take the class for them, so I take it. My boss even knows that if a class gets added late and he needs someone to work it, if I'm available, he calls me and of course I take it, because I always take it. I'm trustworthy and hard working, so pass the buck to Amy, she will take care of it. But as I struggle maybe once every two months or so to find someone to take one of my classes because something comes up, it makes me wonder why? Why won't anyone else take my class when I am always so willing to take theirs? Is it selfishness? Is it laziness? Is is just plain rudeness? I don't know, but it surely bothers me. Unfortunately it is just another unfair thing in my life that there is nothing I can do anything about.