Would you agree that I, not being a trained firefighter, could use the reasoning "I'm not obligated to help" when I see a burning building?

Well the thing is, even a trained firefighter isn't "obligated to help." Basically, I think the whole argument of "obligation" is arbitrary and irrelevant to the actual moral standing of a situation.

For example, you not being a firefighter, nobody would have an expectation on you to put out a fire. You don't have the experience, you could potentially get seriously hurt yourself, you could cause more injuries, etc. So you're not obligated to nor are you really expected to. But if a team of firefighters showed up to a burning house and said "...well fuck that, that's too big to deal with. Let's go home boys" then that's kind of a problem. Sure it's not their obligation to put out the fire, but there's an expectation of them in place there. Same with police, they don't have to come stop the domestic violence or whatever may be going on but if the operator said "sorry we don't want to deal with that right now" and just hung up there would be outrage.

So I guess it has to do with the level of expectation of the person as well. Back to the social situations - when talking to someone you're interested in, you expect some sort of mutual respect for each other. Even if it's not a relationship yet, you'd be inclined to figure hey we've gone on a couple dates she must have some sort of interest in me as a person. But then after two or three dates and they just all of a sudden start ignoring you, that's really a kick in the ass knowing they didn't even respect you enough to be worth a text message. It's very clear to them that you were trying to build a relationship yet they said "eh I don't give a damn about it, I'm gonna ignore him/her instead" and that attitude is inconsiderate and disrespectful regardless of whether or not they "owe an explanation."