Both are thankless jobs, but it seems like healers by their nature take the heat for deaths. Tanks can't save DPS from their own stupidity in the long term, while healers often can. It's a lot of responsibility and it is quite often boring. Usually your eyes are glued to a few bars instead of having fun swinging a sword or ax.

For some reason tanks are just higher in value. I've started playing a priest and it's a nice warm fuzzy to be wanted in groups, but I'm no where near as desirable as tanks are. One tank in my guild got offered thousands of gold to defect and tank for another guild. He's not even 70 yet (and they'd probably regret that purchase after seeing his current skills). I think tanks are put on a pedestal because there are so few of them at the moment, and they really do dictate the pace of fights - healers, just make sure the fight doesn't end prematurely.

As for feeling obligated to tank... just don't. I see the pressure put on people to tank, and I can't agree with it. People in my guild know that if I'm not in the mood they better back off or I will severely flame them. I have 2 un-guilded alts, and they don't know who they are but they know I have them and if the pressure gets passive aggressive then I just have fun on those characters. This is a game not a job!

This is why I am sort of apprehensive to start a tank. When the expansion comes out I'll start a death knight and be one of the millions of DK tanks out there, so I won't be pestered with whispers. That's one of the things I'm looking forward to in the expansion: more tanks and less pressure on the current ones.