It is God who places love in people's hearts

In many verses, God reveals that it is He Who places love and affection in the hearts of people. For example, God has stated in the verse below that it is He Who gathers believers and unites their hearts as brothers:
 

Hold fast to the rope of God all together, and do not separate. Remember God's blessing to you when you were enemies and He joined your hearts together so that you became brothers by His blessing. You were on the very brink of the pit of Fire and He rescued you from it. In this way God makes His Signs clear to you, so that hopefully you will be guided. (Surah Ali 'Imran: 103)
 

In other verses, God informs us that it is He Who gives believers tenderness and compassion.
 

... We gave him judgement while still a child, and tenderness and purity from Us - he had fear of God. (Surah Maryam: 12-13)

As for those who believe and do righteous acts, The All Merciful will bestow His love on them. (Surah Maryam: 96)

Among His Signs is that He created spouses for you of your own kind so that you might find tranquillity in them. And He has placed affection and compassion between you. There are certainly Signs in that for people who reflect. (Surat ar-Rum: 21)
 

God also states that He will place love between believers and those who are hostile to them. It is evident that God holds control over all hearts - both of believers and all other people.
 

It may well be that God will restore the love between you and those of them who are now your enemies. God is All-Powerful. God is Ever-Forgiving, Most Merciful. (Surat al-Mumtahana: 7)