This verse repeats briefly what the author has already stated in verse 10.

We love: in the present verse the pronoun we has some emphasis and contrasts with he in the next clause. The Greek verb form may be taken as indicating a statement (Revised Standard Version and several other versions) or as an exhortation, ‘let us love’ (Zürcher Bibel, Bible de Jérusalem), preferably the former.

If a direct object must be added, it may be a reference to God, or to the brothers (which may lead to a reciprocal expression), or to both. The third possibility seems to be preferable.

He first loved us, or ‘he as the first (of the two agents mentioned) loved us,’ ‘he loved us before we did.’ The sequence of this and the preceding clause is sometimes better reversed; for example, ‘God loved us first; that is why we love (him and the brothers).’

Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .