Unwarranted guilt

I'm such a pleaser. Sometimes it's fine. I actually do like to help other people. But why do I feel guilt when I say no?

Yesterday afternoon, the manager at the bookstore called me with a nicely convoluted story about how they were a person short. I had plans for the evening, and I truly didn't want to go in. We had dinner plans with friends, two stops to see more friends, and family time all planned out for the evening. Nothing was unbreakable, but really I just didn't want to. I asked Skeeter's advice, and he just asked the basic question, "Do you want to go in?" No, I didn't. So I called and told them that I couldn't come in.

Another example: on Monday I was supposed to keep a friend's children during the morning so she could see a relative in the hospital. She didn't call, but I stuck around home thinking that she'd be over any time. I finally called around 1:30, and she hadn't made it out yet. She asked if she could bring them over around 2:30, and I had to say no. We had errands to run that day that I had already put off because I had thought they were coming earlier. And I was working that night, so I had to get the errand running done before 4:30.

So why did I feel selfish in saying no in those instances? I still haven't figured it out.