I want you to know that a man is considered pleasant if his manners conform to the common practices between friends, whereas someone who is eccentric will, in all situations, appear to be a stranger, that is, alien. On the contrary, men who are affable and polite will appear to have friends and acquaintances wherever they may be.

[E sappi che colui è piacevole i cui modi sono tali nell’usanza comune, quali costumano di tenere gli amici infra di loro, là dove chi è strano pare in ciascun luogo «straniero», che tanto viene a dire come «forestiero»; sì come i domestici uomini, per lo contrario, pare che siano ovunque vadano conoscenti et amici di ciascuno.]

Giovanni della Casa
Giovanni della Casa (1503-1556) Florentine poet, author, diplomat, bishop
Galateo: Or, A Treatise on Politeness and Delicacy of Manners [Il Galateo overo de’ costumi], ch. 9 (1558) [tr. Einsenbichler/Bartlett (1986)]
    (Source)

(Source (Italian)). Alternate translations:

And you must understand, that he is pleasaunt and courteous: whose manners bee suche in his common behaviour, as practise to keepe, and maintaine him friendeship amongst them: where hee that is solleyne and way warde, makes him selfe a straunger whersoever hee comes: a straunger, I meane, as much as a forreigne or alienborne.
[tr. Peterson (1576)]

We ought to esteem him alone an agreeable and good-natured man, who, in his daily intercourse with others, behaves in such a manner as friends usually behave to each other. For as a person of that rustic character appears, wherever he comes, like a mere stranger: so, on the contrary, a polite man, wherever he goes, seems as easy as if he were amongst his intimate friends and acquaintance.
[tr. Graves (1774)]