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)