Because the first ionisation energy of the alkali metals decreases down the group, it is easier for the outermost electron to be removed from the atom and participate in chemical reactions, thus increasing reactivity down the group.
bɪkˈʌz ðə fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒi ʌvðɪ ˈælkəlˌaɪ mˈɛɾəlz dˈiːkɹiːsᵻz dˌaʊn ðə ɡɹˈuːp, ɪɾ ɪz ˈiːziɚ fɚðɪ ˈaʊɾɚmˌoʊst ᵻlˈɛktɹɑːn təbi ɹᵻmˈuːvd fɹʌmðɪ ˈæɾəm ænd pɑːɹtˈɪsᵻpˌeɪt ɪn kˈɛmɪkəl ɹɪˈækʃənz, ðˈʌs ɪŋkɹˈiːsɪŋ ɹɪɐktˈɪvᵻɾi dˌaʊn ðə ɡɹˈuːp.