Pressured by making statements in public and wanting to act consistently with him.

Robert Cialdini describes this in some of his awesome books on social psychology.

Basically, if you make a statement in public you are much more likely to act consistently with it.

So in this case:

1) She says that the gift is really cool on Reddit.

2) People say that she's evidently not sold on him, and she updates her post to say that she does like the guy. After all, she's received a cool gift. If she didn't like the guy, why would she think it was cool and not creepy?

3) She's now committed in public that a) she thinks the gift is awesome and b) she likes the guy. Under those circumstances making the trip is the consistent thing to do.

4) Hence she's now making a fourteen hour flight to see him.

On the other hand, if she hadn't posted it there'd be no pressure to assert that she liked the guy and no consequent pressure to make the trip. She'd probably think "neat gift, but he's fourteen hours away" and then find reasons not to go.

She considers herself a member of a community; according to the mores of that community there's a clearly favored course of action, although she might not have realized that before posting.