Shaykh Muhammad al-Saalih al-'Uthaymeen (may Allaah have mercy on him) was asked about the ruling on mixing with the kuffaar [unbelievers, non-Muslims] and treating them kindly hoping that they will become Muslim. He replied:

Undoubtedly the Muslim is obliged to hate the enemies of Allaah and to disavow them, because this is the way of the Messengers and their followers...

Based on this, it is not permissible for a Muslim to feel any love in his heart towards the enemies of Allaah who are in fact his enemies too...

But if a Muslim treats them with kindness and gentleness in the hope that they will become Muslim and will believe, there is nothing wrong with that, because it comes under the heading of opening their hearts to Islam. But if he despairs of them becoming Muslim, then he should treat them accordingly. This is something that is discussed in detail by the scholars, especially in the book Ahkaam Ahl al-Dhimmah by Ibn al-Qayyim (may Allaah have mercy on him)...