It would never really bum me out. Probably because for me, I have places where I post comics to get the kind of comments and favs. Tapastic to me was the place I could finally do what I love and actually get a little money out of it as well. If I found out someone subbed out of obligation (or more importantly subbed in the hopes of receiving something back) I'd be rather indifferent.
It would be different if someone said, "Sir, your comic sucks, I can't believe I subbed out of good will..." That would feel more like pity than perhaps motivational.
I myself don't sub in the hopes of getting something back. if I sub to something it's either because I like the series, think I will like it or have received something in return and feel the need to give them a "pat on the back" of sorts. I'm the kind of person who likes to compromise with others who have strongly been supportive of me.
A lot of my readers, not only here but on other hosting sites have been very supportive of me and my work. I'm almost always trying to support them. So when I receive a sub, while not always I do try to do something to say thanks and giving them a sub basically says to me:
"You took the time to support me, let me take the time to support you and hopefully that support will blossom into something larger."