I didn't really think Severus was only thinking of himself in that line. He'd already been told Harry would have to sacrifice himself after hunting down all the horcruxes. Of course, you don't learn that until later in the story. Personally, I think Severus did sacrifice more than anyone else bar Harry and at times I think he's entitled to be a little bitter about it. He's in a bit of snit over this latest thing so he probably is even more snappish than usual!!
As for the Dumbledore line, I have to say - I loved when that line came out of me. Sometimes I think these things write themselves. I typed, "I only had to die" before I even realized what I had written. As soon as I did though, I sat back and went 'Yup, I like that'. More often than not though crap comes out first and I have to edit deeply.