Jamieson, Fausset and Brown on Romans 12:10

10. Be, &c.--better, "In brotherly love be affectionate one to another; in [giving, or showing] honor, outdoing each other." The word rendered "prefer" means rather "to go before," "take the lead," that is, "show an example." How opposite is this to the reigning morality of the heathen world! and though Christianity has so changed the spirit of society, that a certain beautiful disinterestedness and self-sacrifice shines in the character of not a few who are but partially, if at all under the transforming power of the Gospel, it is only those whom "the love of Christ constrains to live not unto themselves," who are capable of thoroughly acting in the spirit of this precept.