In Chapter 9 of the Nichomachean Ethics Aristotle defends - with a rather tortured argument - the claim that a virtuous friend is naturally desirable for a virtuous individual. I’ve attempted to extract his argument here (Let “John” and “Sally” designate distinct virtuous individuals):

(1) John exists
(2) If x exists then x perceives/thinks
(3) If x perceives/thinks then x perceives that x perceives/thinks
(4) If x perceives that x perceives/thinks then x perceives that x exists
(5) Hence, if x exists then x perceives that x exists                                                                                                     (from 2-4)
(6) John perceives that John exists                                                                                                                                     (from 1,4)
(7) If x exists then x's existence is intrinsically good/pleasant for x
(8) John's existence is intrinsically good/pleasant for John                                                                                    (from 1,6,7)
(9) If z is intrinsically good/pleasant for x & x perceives z, z seems good/pleasant for x
(10) Hence, John's existence seems good/pleasant for John                                                                                    (from 6,8,9)
(11) If y is x's friend, then whatever is intrinsically good/pleasant for x is intrinsically good/pleasant for y
(12) Sally is John's friend
(13) Hence, whatever is intrinsically good/pleasant for John is intrinsically good/pleasant for Sally   (from 11,12)
(14) Hence, John's existence is intrinsically good/pleasant for Sally                                                                   (from 8-13)
(15) If y is x's friend, then whatever seems intrinsically good/pleasant for x seems intrinsically good/pleasant for y
(16) Hence, John's existence seems intrinsically good/pleasant for Sally                                                          (from 12,15)
(17) If y is x's friend, then x is y's friend
(18) Hence, John  is Sally's friend                                                                                                                                         (from 12,17)
(19) Hence, Sally's existence is intrinsically good/pleasant for Sally                                                                   (repeat 1-8 replacing "John" with "Sally")
(20) Hence, Sally's existence seems intrinsically good/pleasant for Sally                                                         (repeat 1-10 replacing "John" with "Sally")
(21) Hence, whatever is intrinsically good/pleasant for Sally is intrinsically good/pleasant for John   (from 11,18)
(22) Hence, Sally's existence is intrinsically good/pleasant for John                                                                   (from 19-21)
(23) Hence, Sally's existence seems intrinsically good/pleasant for John                                                          (from 15,18)
(24) If z is intrinsically good/pleasant for y & z seems intrinsically good/pleasant for y, then z is desirable to y
(25) Hence, Sally's existence is desirable to John                                                                                                          (from 22,23,24)
(26) Hence, John's existence is desirable to Sally                                                                                                          (from 14,16,24)