The obstacle doesn't have to be external, it could be that the boy is cripplingly shy and has to overcome that in order to achieve his goal of getting the girl to like him. The point is that there is still a conflict at the core of the concept.
I do agree that you don't need to know the concept when you first put fingers to keyboard (or pen to paper) You might start with a moment, a single situation, and expand from that and through writing work out what the core conflict is (and it might well be someone very different from originally imagined).
Yes, I think this is exactly right. It seems in genre fiction, however, and perhaps even more in new writers to genre fiction, there is an assumption that a dramatic external obstacle is needed. I took BWFoster's example to mean that you can write a great story of 'boy wants girl' based just on the emotions and tensions of the characters themselves. You still need some kind of conflict, but it doesn't have to be orcs sweeping out of the hills to kidnap the girl.