Fluoride "loses" a pair of valence electrons because the electrons shared in the B—F bond are located in the region of space between the two atomic nuclei and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion.
flˈʊɹɹaɪd "lˈuːzᵻz" ɐ pˈɛɹ ʌv vˈeɪləns ᵻlˈɛktɹɑːnz bɪkˈʌz ðɪ ᵻlˈɛktɹɑːnz ʃˈɛɹd ɪnðə bˈiː—ˈɛf bˈɑːnd ɑːɹ loʊkˈeɪɾᵻd ɪnðə ɹˈiːdʒən ʌv spˈeɪs bᵻtwˌiːn ðə tˈuː ɐtˈɑːmɪk nˈuːklɪi ænd ɑːɹ ðˈɛɹfoːɹ mˈoːɹ dˈɪstənt fɹʌmðə flˈʊɹɹaɪd nˈuːklɪəs ðɐn ðeɪ ɑːɹ ɪnðə lˈoʊn flˈʊɹɹaɪd ˈaɪɑːn.