Yo not only is this Dipper smart and cute and interesting, he's loyal. And genuine loyalty is something Other Bill hasn't experienced in a long time (probably).

In my headcanon, it's a thing that neither Bill has experienced in a long time - if ever. Which makes it compelling! I mean, imagine:

A person who isn't after favors, or power - who doesn't care about your position in the world, whether they think you're some middling jobber of a demon or a literal Nightmare King. Who doesn't want you to do stuff for them, because they've got a stubborn independent streak. Someone who laughs at your jokes only when they actually think they're funny. Who seduces you not because of the advantages it could give them, but because they honestly want to bed you. Someone who disagrees with you - will argue with you - and while there are fundamental differences, it doesn't turn them off.

Despite the fact that they should hate you, or avoid you, do the sensible thing for their species - even with so many escape routes available, they aren't leaving.

Because this someone genuinely, honestly, likes you.

Just because you're... you.

Throw in all the other stuff about Dipper, and Our Bill never stood a chance.