In terms of form, indicative or subjunctive are possible. Others, especially Carl, have already given abundant reasons for why the indicative is far more likely. So, I will not repeat the reasons already given (to which I concur) & will only concentrate on making some additional points.
First, the most relevant reading out of possible readings should usually be preferred. Second, careful attention to the flow of the discourse & to how a theme is developed in a book is one of the most natural ways to determine the most relevant reading. I think most interpreters (including most contributors to this forum) would naturally select the indicative as the most relevant reading, even if explaining all the reasons why may be difficult to do & would take a lot of time to communicate (whereas in natural reading the decision is made in a split second). Third, for 1 John in particular, it is important to notice (a) how the author interweaves assertions of reality, obligation, and exhortation; & (b) how the author cycles through various topics & exhortations, always with progression on some aspect while tying more themes together as the book progresses.
With love, note that in:
2:5 there is an assertion of reality: whoever keeps his word, in him truly the love of God has reached its goal (ὃς δʼ ἂν τηρῇ αὐτοῦ τὸν λόγον, ἀληθῶς ἐν τούτῳ ἡ ἀγάπη τοῦ θεοῦ τετελείωται)
2:10 there is an assertion of reality: the one who loves his brother/sister abides in the light (has fellowship with God) (ὁ ἀγαπῶν τὸν ἀδελφὸν αὐτοῦ ἐν τῷ φωτὶ μένει)
3:1 there is an assertion of reality: the great love God has given Christians that they should be called children of God (Ἴδετε ποταπὴν ἀγάπην δέδωκεν ἡμῖν ὁ πατήρ, ἵνα τέκνα θεοῦ κληθῶμεν, καὶ ἐσμέν.)
In the previous sustained treatment of love in 3:11-24, a summary may be:The author reminds his readers that they should love one another (3:11), unlike Cain (3:12), that they shouldn’t be surprised that the world hates them (3:13), & that they shouldn’t just claim to love, but truly love in practice (3:18), knowing that loving is a sure sign of having been transferred from death to life (3:14). He also reminds them that love shows itself in laying down our lives for our brothers & sisters (3:16) & showing compassion & meeting their needs (3:17). We know that we belong to the truth & can set our hearts at rest in God’s presence when our hearts don’t condemn us (3:19-20) & that we can have confidence before God & receive whatever we ask because we obey God’s commands (3:21-22) to believe in the name of his Son & to love one another (3:23). In this section, the nature of love as a command and its presence as giving knowledge of the reality of possessing eternal life are picked up from 2:5 & 2:10 & developed further in conjunction with other themes from the book. Note that love is a command (3:11), it is a reality already present in Christians, which allows them to know that they have eternal life (3:18), & it is also an ongoing obligation that sustains that reality of eternal life (3:23). (11 Ὅτι αὕτη ἐστὶν ἡ ἀγγελία ἣν ἠκούσατε ἀπʼ ἀρχῆς, ἵνα ἀγαπῶμεν ἀλλήλους, 12 οὐ καθὼς Κάϊν ἐκ τοῦ πονηροῦ ἦν καὶ ἔσφαξεν τὸν ἀδελφὸν αὐτοῦ· καὶ χάριν τίνος ἔσφαξεν αὐτόν; ὅτι τὰ ἔργα αὐτοῦ πονηρὰ ἦν τὰ δὲ τοῦ ἀδελφοῦ αὐτοῦ δίκαια. 13 [Καὶ] μὴ θαυμάζετε, ἀδελφοί, εἰ μισεῖ ὑμᾶς ὁ κόσμος. 14 ἡμεῖς οἴδαμεν ὅτι μεταβεβήκαμεν ἐκ τοῦ θανάτου εἰς τὴν ζωήν, ὅτι ἀγαπῶμεν τοὺς ἀδελφούς· ὁ μὴ ἀγαπῶν μένει ἐν τῷ θανάτῳ. 15 πᾶς ὁ μισῶν τὸν ἀδελφὸν αὐτοῦ ἀνθρωποκτόνος ἐστίν, καὶ οἴδατε ὅτι πᾶς ἀνθρωποκτόνος οὐκ ἔχει ζωὴν αἰώνιον ἐν αὐτῷ μένουσαν. 16 ἐν τούτῳ ἐγνώκαμεν τὴν ἀγάπην, ὅτι ἐκεῖνος ὑπὲρ ἡμῶν τὴν ψυχὴν αὐτοῦ ἔθηκεν· καὶ ἡμεῖς ὀφείλομεν ὑπὲρ τῶν ἀδελφῶν τὰς ψυχὰς θεῖναι. 17 ὃς δʼ ἂν ἔχῃ τὸν βίον τοῦ κόσμου καὶ θεωρῇ τὸν ἀδελφὸν αὐτοῦ χρείαν ἔχοντα καὶ κλείσῃ τὰ σπλάγχνα αὐτοῦ ἀπʼ αὐτοῦ, πῶς ἡ ἀγάπη τοῦ θεοῦ μένει ἐν αὐτῷ; 18 Τεκνία, μὴ ἀγαπῶμεν λόγῳ μηδὲ τῇ γλώσσῃ ἀλλὰ ἐν ἔργῳ καὶ ἀληθείᾳ. 19 [Καὶ] ἐν τούτῳ γνωσόμεθα ὅτι ἐκ τῆς ἀληθείας ἐσμέν, καὶ ἔμπροσθεν αὐτοῦ πείσομεν τὴν καρδίαν ἡμῶν, 20 ὅτι ἐὰν καταγινώσκῃ ἡμῶν ἡ καρδία, ὅτι μείζων ἐστὶν ὁ θεὸς τῆς καρδίας ἡμῶν καὶ γινώσκει πάντα. 21 Ἀγαπητοί, ἐὰν ἡ καρδία [ἡμῶν] μὴ καταγινώσκῃ, παρρησίαν ἔχομεν πρὸς τὸν θεὸν 22 καὶ ὃ ἐὰν αἰτῶμεν λαμβάνομεν ἀπʼ αὐτοῦ, ὅτι τὰς ἐντολὰς αὐτοῦ τηροῦμεν καὶ τὰ ἀρεστὰ ἐνώπιον αὐτοῦ ποιοῦμεν. 23 Καὶ αὕτη ἐστὶν ἡ ἐντολὴ αὐτοῦ, ἵνα πιστεύσωμεν τῷ ὀνόματι τοῦ υἱοῦ αὐτοῦ Ἰησοῦ Χριστοῦ καὶ ἀγαπῶμεν ἀλλήλους, καθὼς ἔδωκεν ἐντολὴν ἡμῖν. 24 καὶ ὁ τηρῶν τὰς ἐντολὰς αὐτοῦ ἐν αὐτῷ μένει καὶ αὐτὸς ἐν αὐτῷ· καὶ ἐν τούτῳ γινώσκομεν ὅτι μένει ἐν ἡμῖν, ἐκ τοῦ πνεύματος οὗ ἡμῖν ἔδωκεν.)
The reason I went into this survey of how the theme of love is developed earlier in 1 John is to show that the author develops his theme more subtly than simply repeating exhortations. As we shall see below, the next major treatment of love in 4:7-21 fits this pattern.
A summary of 4:7:21 may be: The author calls his readers to love one another (4:7, 11) because God is love (4:8, 16) & the source of love (4:7) & showed his love by sending his Son as an atoning sacrifice for our sins (4:9-10) & God’s love is made complete in them when they love one another (4:12). He reminds them that they know that they live in God & he in them by the Spirit he gave them (4:13) & that they know & rely on the love God has for them (4:16), & that God has given them this command: Whoever loves God must also love his brother/sister (4:21). Having this love made complete in them, in fact, drives out fear (4:17-18) & gives confidence on the day of judgment. (7 Ἀγαπητοί, ἀγαπῶμεν ἀλλήλους, ὅτι ἡ ἀγάπη ἐκ τοῦ θεοῦ ἐστιν, καὶ πᾶς ὁ ἀγαπῶν ἐκ τοῦ θεοῦ γεγέννηται καὶ γινώσκει τὸν θεόν. 8 ὁ μὴ ἀγαπῶν οὐκ ἔγνω τὸν θεόν, ὅτι ὁ θεὸς ἀγάπη ἐστίν. 9 ἐν τούτῳ ἐφανερώθη ἡ ἀγάπη τοῦ θεοῦ ἐν ἡμῖν, ὅτι τὸν υἱὸν αὐτοῦ τὸν μονογενῆ ἀπέσταλκεν ὁ θεὸς εἰς τὸν κόσμον ἵνα ζήσωμεν διʼ αὐτοῦ. 10 ἐν τούτῳ ἐστὶν ἡ ἀγάπη, οὐχ ὅτι ἡμεῖς ἠγαπήκαμεν τὸν θεὸν ἀλλʼ ὅτι αὐτὸς ἠγάπησεν ἡμᾶς καὶ ἀπέστειλεν τὸν υἱὸν αὐτοῦ ἱλασμὸν περὶ τῶν ἁμαρτιῶν ἡμῶν. 11 Ἀγαπητοί, εἰ οὕτως ὁ θεὸς ἠγάπησεν ἡμᾶς, καὶ ἡμεῖς ὀφείλομεν ἀλλήλους ἀγαπᾶν. 12 θεὸν οὐδεὶς πώποτε τεθέαται. ἐὰν ἀγαπῶμεν ἀλλήλους, ὁ θεὸς ἐν ἡμῖν μένει καὶ ἡ ἀγάπη αὐτοῦ ἐν ἡμῖν τετελειωμένη ἐστίν. 13 Ἐν τούτῳ γινώσκομεν ὅτι ἐν αὐτῷ μένομεν καὶ αὐτὸς ἐν ἡμῖν, ὅτι ἐκ τοῦ πνεύματος αὐτοῦ δέδωκεν ἡμῖν. 14 καὶ ἡμεῖς τεθεάμεθα καὶ μαρτυροῦμεν ὅτι ὁ πατὴρ ἀπέσταλκεν τὸν υἱὸν σωτῆρα τοῦ κόσμου. 15 Ὃς ἐὰν ὁμολογήσῃ ὅτι Ἰησοῦς ἐστιν ὁ υἱὸς τοῦ θεοῦ, ὁ θεὸς ἐν αὐτῷ μένει καὶ αὐτὸς ἐν τῷ θεῷ. 16 καὶ ἡμεῖς ἐγνώκαμεν καὶ πεπιστεύκαμεν τὴν ἀγάπην ἣν ἔχει ὁ θεὸς ἐν ἡμῖν. Ὁ θεὸς ἀγάπη ἐστίν, καὶ ὁ μένων ἐν τῇ ἀγάπῃ ἐν τῷ θεῷ μένει καὶ ὁ θεὸς ἐν αὐτῷ μένει. 17 Ἐν τούτῳ τετελείωται ἡ ἀγάπη μεθʼ ἡμῶν, ἵνα παρρησίαν ἔχωμεν ἐν τῇ ἡμέρᾳ τῆς κρίσεως, ὅτι καθὼς ἐκεῖνός ἐστιν καὶ ἡμεῖς ἐσμεν ἐν τῷ κόσμῳ τούτῳ. 18 φόβος οὐκ ἔστιν ἐν τῇ ἀγάπῃ ἀλλʼ ἡ τελεία ἀγάπη ἔξω βάλλει τὸν φόβον, ὅτι ὁ φόβος κόλασιν ἔχει, ὁ δὲ φοβούμενος οὐ τετελείωται ἐν τῇ ἀγάπῃ. 19 ἡμεῖς ἀγαπῶμεν, ὅτι αὐτὸς πρῶτος ἠγάπησεν ἡμᾶς. 20 ἐάν τις εἴπῃ ὅτι ἀγαπῶ τὸν θεὸν καὶ τὸν ἀδελφὸν αὐτοῦ μισῇ, ψεύστης ἐστίν· ὁ γὰρ μὴ ἀγαπῶν τὸν ἀδελφὸν αὐτοῦ ὃν ἑώρακεν, τὸν θεὸν ὃν οὐχ ἑώρακεν οὐ δύναται ἀγαπᾶν. 21 καὶ ταύτην τὴν ἐντολὴν ἔχομεν ἀπʼ αὐτοῦ, ἵνα ὁ ἀγαπῶν τὸν θεὸν ἀγαπᾷ καὶ τὸν ἀδελφὸν αὐτοῦ.)
The assertions of reality in 2:5, 2:10, & 3:1 are all taken up & tied together in this section. Note again that love is exhorted (4:7, 11) & at the same time it is a present reality that represents the completion of the goal of God's love for Christians (4:12, 17-18), leading to continued fellowship with God & the absence of fear of punishment. 4:19 fits in naturally as an assertion that we love because [God] first loved us.