You lived in Gravity Falls and Stanford Pines is your father, you also knew Bill Cipher, he acted like another father figure to you and Ford didn't like that.. Bill always acted like you were his child around you

Bill: You are like the child that I wished I had.

Ford: Uh, what?

Bill: I care for you just like a daughter/son I spawned. Bill says as he pats your head

Ford: Hold on now-!

Bill: It's a little funny, you can almost call me "Dad"!